The Wales children should be taught Welsh also.
I think possibly George should, yes. William definitely needs to learn some.
I'm being realistic, though. I've had a go at leaning some Welsh and I've found it difficult in an artificial context (I mean not living there and being totally immersed), and if you don't use it a lot, you'll soon lose it. So I think it's unrealistic to expect William to become really fluent, but he could learn some basic, conversational Welsh and be able to make speeches at least.
But (and I'm a mild, non-fawing royalist 😆) I think, diplomatically and politically, the whole Prince of Wales thing should probably quietly fade away in the next generation. Or at least, the Welsh should be asked if they want the office to continue in any form.