Might not like it, but it would be perfectly okay to put your children in the back here in Arizona, as long as all the other seats were filled first.
You can even drive around with 20 people, including children in the back of your pick-up without being bothered by the police.
Alberto, seems like a valid question to me, but since I'm from New Jersey originally, maybe you should have asked, "How many bodies can you fit in the trunk area?" and people would not have worried about it so much. Laws vary from state to state.