Interesting idea but probably much ado about nothing. Although, I do think it would be kind of cool if they got together to try composing some original material as a group and see what happens. I don’t think some kind of extended thing with them performing just Beatles songs would be a good idea. Maybe a project with their own music and throw in a few Beatles covers just for fun? And yes, no one take it TOO seriously just have fun with no high expectations.
I think Zak is closer to Julian’s age?
Personally, I would like Julian better for the project and really, even though he’s almost exactly a year younger than me it’s not like he’s OLD for heaven’s sake! I know ... old in the music business

And who said it has to be one kid per Beatle?

I think there’s room for everyone interested!
I like Ziggy Marley's stuff. I think he had a few family members in his band but wasn't actually the Marley kids band mentioned above.