Do children need to pay for visa extensions in Thailand if they overstay?

I have a USA passport and got a 45-day visa exempt entry which expires Dec 17. I have my wife and 5 children here with me. We are going to pay the 1,900 baht fee to get the 30-day extension this week. I assume the 1,900 baht fee is per person - but it seems that I don't need to pay to extend my 5 children if children can't be fined for overstay, is that correct? We are going to fly to Laos for a border bounce in January, so I want to make sure there won't be any problems if they have overstayed.

Separately, I'm in Nimman area of Chiang Mai and I remember there being some advice about a particular immigration office in Chiang Mai that is not very busy. Any suggestions on which one to go to?