If your child is under a certain age there is no penalty for overstay, but they do get an overstay stamp in their passport. As long as they are with a legal parent/guardian that has proper entry authorization on their next re-entry then no issue.
The Chonburi immigration office in Pattaya has also cracked down on this. They were going to fine my gf something like 2500 baht for not having a current TM.30 when she went in to do her 30 day extension recently. She elected to do a border run instead to restart her clock.