Can I apply for my daughter’s long-term visa under my husband’s retirement visa in Thailand?

Your child can attach to the retirement visa till they are 20 yrs old. Definitely worth doing IMO.

Completely correct that children do not get penalized for overstay when under 15 years old

Well the fact she was born here doesn't cut any ice, but don't worry in the least about having a foreign child that young overstay their free stamps..

I would just let them overstay until you're going to enroll them in school here, (which will be several years from now).

You can indeed get the child 'piggy backed' on your husbands retirement extension, but you would have to go to your immigration office where he got that issued and see what the requirements are for it. I would wager as young as the child is they'll say just let them overstay.

You are correct, there is no overstay fine for children under 15 and children under 18 can't be banned..

I know foreigners here with foreign children on several YEARS overstay and nothing happens when they leave the country and come back.