Can dual nationality children leave Thailand on their UK passport despite entering on expired Thai passports?

OP got his answer. And this is far from this group's scope, commenting is closed now.

No, we have had experience with this

that was a fail bringing them in on the thai expired passports (especially IF there were any anticipated problems renewing them INSIDE thailand)

They should have come in on the brit passports and just gotten year extensions as being thai nationals that entered on another country passport for 1900baht at the immigraiton office with the mothers thai i/d and their expired thai passports 🙁

.

Now as Robert said, there's no way around having to get them a new thai passport. SO if that means getting them on a thai house book, that's what you do, because you HAVE to leave the country you entered on 😮

Good luck to them.

Stretching the boundary of a visa question quite a bit (IMHO) though 😉

No, as they need an entry stamp in their passport to use it to leave the country.