Gaza children deliberately transported to war zones

In the last few weeks, several children have been reported killed in the border clashes with Gaza.  How does this happen?  Why are children in a battle zone, instead of in school?

Sadly, the children are bought to the combat zone deliberately, either by their parents and family members

or in free buses organized by Hamas, who regards the death of children as a win important tool in their efforts to demonize Israel.

It is a war crime to use children as both combatants, and as human shields.