When it's wet I've always just worn trainers as I've found I'm never outside long enough to get that wet. That said, there was never a night time show like Dreams! on when I was there during a wet stay. maybe go back to the hotel and change into wellies for the evening?
I would take two or three pairs of comfy shoes each if you can. When we were there last May it rained really hard on one of the days and my trainers did get soaked through so I was glad of a second pair to wear while the others dried out on the radiator.
Are you near a Millets? As the store near me has a sale on kids walking boots and they r waterproof. Also if you can get them cheap somewhere, the croc wellies for kids r v comfy.