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.
Look, Thailand isn't really a "mother may I" kind of country. Do your thing, don't draw attention to yourself and generally things will sort themselves out, especially with a little bit of tea money to the right people. Just keep your mouth shut.
Be aware though that if you are doing anything outside the perceived norm you are making yourself a target for extortion, both by locals and unfortunately other expats. That's why you keep your mouth shut about whatever it is you are trying to accomplish and make it legit sooner rather than later.
