Can Prince Williams kids become queen?

It is possible. Currently, Prince Charles is first in line for the throne, then Prince William, then Prince Harry. But any of William's children, male or female, will be closer to the throne than Harry. So while it seems even William is a long way away from taking the throne, any children he has will be next in line (a daughter could always lose her place to a newly born male sibling, due to the male bias of the system).