can't set up cellular for apple watch for my kid / new apple watch SE3

You should also report this problem directly to Apple by using the Feedback link, so they may implement a fix in one of the upcoming updates. This is the only way to reach them, Apple does not follow these user to user discussions. The more users do this, the higher the report will be ranked in the watchOS issue list for the Apple Techs.

Feedback - Apple Watch - Apple

We have exactly the same issue here in Australia. Better roaming the only provider. Apple Support kept on trying to say the issue is with better roaming until they agreed that they have a software issue but still have not heard anything from them.

The products are not cheap as is, but ludicrously expensive if they are useless! I will follow your thread too in the hope that one of us gets an answer! Good luck

Hi Antoine, ours suddenly worked this morning. We did not have to run any updates but I wiped the watch again last night and set up fresh. It did not work last night but this morning it connected the cellular. Hope the same goes for you.