Shaykhul Islaam ibn Taymiyyah (رحمه الله) was asked:

Regarding the believing children (on the day of judgement), will they remain in the state that they died in? Or will they be older and marry? And also the girls, will they marry?

Answer: All praise is due to Allaah. If they enter Jannah (paradise), then they will enter as the adults will enter it, in the shape of their father Adam and marry as the adults marry.

And the ones who died from the girls and did not marry (in this life), then they will be married in the hereafter.

And also like them, the boys who died (without marrying), then they will be married in the hereafter.

And Allaah, the Most High, knows best.