Do children under the age of 15 need a visa to enter or leave Thailand if their parents have a valid visa?

It's a common misconception that children don't require a visa..

The RULE is actually children under the age of 15 can't be FINED for overstay and children under the age of 18 can't be BANNED for overstay.

There is no negative repercussions for the children OR the parents when children under 15 overstay. I know foreigners with children here who were on multi-YEAR overstays, who exit/re-enter the country without problem. They even attend school here on overstay..

Now with that being said what children can't do once they go on overstay is get ANY visa issued inside the country. They'd have to exit/re-enter to clear the overstay and come in on a new entry stamp to do that.

AND

Parents of foreign children here can't use children on overstay to get a "guardian visa", because to get that visa the child has to be legally enrolled in a school on an ED visa

You really should get the children the correct visas, which I would hazard a guess would be Education visas?