grockle90

I've also come across references saying it's interchangeable for movement 4U13 (which is apparently an older one, although both now obsolete).

The 4U13 uses a 321/SR616SW battery, same height but smaller diameter.

Might be able to do it with a rubber ball, depends how tight it's on there vs how strong a grip the ball can get. Professionally, this would be done with something like https://www.cousinsuk.com/product/rolex-dies-6-with-wrench

Failing that, it's the same type of back as Rolex, so look up how to open Rolex style case backs for other ideas.

And stupid question, have you checked it's not a snap on pretending to be a screw on?

I'd definitely suggest a Lorus

https://www.loruswatches.com/Products/Kids

Stand up to a decent amount of daily wear, pricewise definitely in the "doesn't matter too much if lost/broken" which you've got to consider with kids.

And until I checked just now, I didn't realise how much of a range they actually have for kids watches.