The UK based CTC bike forum would probably be a better place to ask, although I suspect that like here, most of the users are older folks not with young children.

I suspect though that you might get better suggestions from the UK crowd for possible other ideas.

Just be aware of course how hard it is already doing stuff with kids (as you know of course) so coordinating with another family does add another level of complexity, not even touching on the social / personalities / abilities and all that human stuff.