Thanks for your question.
Nobody will be raised from the dead who has not first come to the Lord Jesus Christ in the way God has appointed. Our Lord says that belief in the Gospel is a pre-requisite to salvation and that the sign of one's positive response to the Gospel and entry into a covenant relationship with the Father through him is baptism (see Acts 8:12; Mark 16:15,16; Romans 6; Galatians 3:27-29). However, although we may not be able to see how it would be possible according to our finite minds and given the information available on the subject in the Bible, perhaps some will be saved outside of these criteria. All things are possible with God.
Will the deceased children of believers be raised to live out their mortal lives in the Millennial kingdom? If God wills it then it will be so. However, we must not presuppose to mandate as doctrine that about which scripture is silent. Either He will or He will not. If not, then we will understand perfectly why not and in that understanding be content.
So, as far as we can tell from scripture those who do not know the gospel or who are incapable of understanding it and responding to it including very young children, will not be raised given that belief, repentance and baptism are stated pre-requisites for resurrection and salvation. However, if the Judge of all the earth decides to make some exceptions, that is His prerogative but it is not something about which we may be presumptuous. This teaching, to my mind, lends urgency to preaching both in outreach by believers and in the need to respond on the part of those who hear.
Here is some information on the https://www.thisisyourbible.com website which will be valuable for you to read regarding this question:
Why Baptism Really Matters
I hope you have found this helpful.