Thanks for your question.
The first time an angel is introduced in the Old Testament is an appearance to Hagar, the Egyptian handmaid who was to bear Abraham’s first son Ishmael.
Genesis 16:7 And the angelof the LORD found her by a fountain of water in the wilderness, by the fountain in the way to Shur.
11 And the angelof the LORD said unto her, Behold, thou art with child, and shalt bear a son, and shalt call his name Ishmael; because the LORD hath heard thy affliction.
14 years after the birth of Ishmael, Isaac, the child of promise, was born to Sarah and on the day that he was weaned Ishmael (about 17-19 years old) was seen to ‘mock’ Isaac which was of serious concern to Sarah
Genesis 21:8 And the child grew, and was weaned: and Abraham made a great feast the same day that Isaac was weaned.
9 And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking.
10 Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac.
11 And the thing was very grievous in Abraham’s sight because of his son.
12 And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.
13 And also of the son of the bondwoman will I make a nation, because he is thy seed.
14 And Abraham rose up early in the morning, and took bread, and a bottle of water, and gave it unto Hagar, putting it on her shoulder, and the child, and sent her away: and she departed, and wandered in the wilderness of Beersheba.
15 And the water was spent in the bottle, and she cast the child under one of the shrubs.
16 And she went, and sat her down over against him a good way off, as it were a bowshot: for she said, Let me not see the death of the child. And she sat over against him, and lift up her voice, and wept.
17 AndGod heard the voice of the lad; and the angelof God called to Hagar out of heaven, and said unto her, What ailed thee, Hagar? fear not; for God hath heard the voice of the lad where he is.
18 Arise, lift up the lad, and hold him in thine hand; for I will make him a great nation.
19 And God opened her eyes, and she saw a well of water; and she went, and filled the bottle with water, and gave the lad drink.
20 AndGod was with the lad; and he grew, and dwelt in the wilderness, and became an archer.
21 And he dwelt in the wilderness of Paran: and his mother took him a wife out of the land of Egypt.
We can conclude that God was concerned with Abraham’s children from before their conception and angels were active in their lives from birth.
When we come to the New Testament
Matthew 18:1 At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven?
2 And Jesus called a little child unto him, and set him in the midst of them,
3 And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
4 Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.
5 And whoso shall receive one such little child in my name receiveth me.
6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.
10 Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angelsdo always behold the face of my Father which is in heaven.
Now it might be argued that ‘these little ones’ is no longer referring to children but to adults who have humbled themselves to become as a little child (ie teachable) but in the foreknowledge of God the distinction is superfluous for God has known from before the foundation of the world who will humble themselves to receive the Gospel of the Kingdom of God.
Eph 1:4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
Mt 25:34 Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world:
Mt 19:13 Then were there brought unto him little children, that he should put his hands on them, and pray: and the disciples rebuked them.
14 But Jesus said, Suffer little children, and forbid them not, to come unto me: forof such is the kingdom of heaven.
If we accept that God knows the end from the beginning and that we still have freewill to accept or reject Him, but He knew before we were born what we will do, then we can come to grips with the doctrine of ‘election’ (Greek from eklegomai – to choose).
1Peter 1:1 Peter, an apostle of Jesus Christ, to the strangers scattered throughout Pontus, Galatia, Cappadocia, Asia, and Bithynia,
2 Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you, and peace, be multiplied.
We are told that the angels minister to ‘those who shall be heirs of salvation’ although we have little idea how their influence affects our lives in practice. It is no guarantee of freedom from disease, longevity of life etc. as the ministering is for spiritual and not natural wellbeing.
Heb 1:13 But to which of the angelssaid he at any time, Sit on my right hand, until I make thine enemies thy footstool?
14 Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?
As we have seen angels had charge over Abraham’s seed (children). Those that are baptised into Christ become Abraham’s seed by adoption.
Galatians 3:29 And if ye be Christ’s, then are ye Abraham‘s seed, and heirs according to the promise.
I think we can conclude that angels are active from the conception of those who areelect according to the foreknowledge of God and are ordained to eternal life in the Kingdom of God; but we have no way of knowing in our lifetime how they work or to whom this applies.
We cannot, however, conclude that all babies are given into the charge of angels, because many die before they have any understanding and many are born into ‘cultures’ that exclude the God of the Bible and will not recognise Christ.
The Gospel hope is notuniversal salvation. The vast majority live and die to perish forever.
Matthew 7:13 Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:
It is only by understanding and obeying the Gospel that one can have hope of eternal life.
Paul tells us that children are sanctified by their parents and certainly the children of Abraham benefited because Abraham was their father.
1Corinthians 7:14 For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy.
The best we can do for our children is to believe and obey the gospel ourselves and take Paul’s advice to the Ephesians.
Ephesians 6:4 And, ye fathers, provoke not your children to wrath: but bring them up in the nurture and admonition of the Lord.
In the fullness of time they will have to make their own choice whether to serve God or not.