When we look into Scripture, we see that those who are in heaven have some sort of heavenly body.  They are seen standing, wearing clothes, holding things in their hands, and speaking; things that a very young child could not do.¹  Even so, we know that very young children are a part of the kingdom of heaven.²

In answer to your question, there are those who believe that children who go to heaven grow into maturity over time. Their belief (to a large degree) is based on what Christ Jesus said of children: "Let the little children come to Me, and do not hinder them, for the kingdom of heaven belongs to such as these."³  In saying this, Christ Jesus was not saying that young children who go to heaven remain as children, but, rather, was saying that many young children, in childlike faith, come to a saving faith in Him.

On the other hand, there are those (myself included) who believe that, just as we are like Christ Jesus in true holiness when we get to heaven, we are also like Him in appearance (around 33 years of age.) ⁴  This is true of all who go to heaven; no matter what age they are when they die.

Bible References:

¹ (Revelation 7:9-11)

² (Matthew 19:14; 2 Samuel 12:22-23)

³ (Matthew 19:14)

⁴ (1 John 3:2b; Philippians 3:20-21)