You can be as prepared as you like

But if they won't wear the waterproofs (or boots or even shoes) then you're going to have a lot of wet (and sandy) clothes.

Lots of them.

One lot from playing in the sandpit in that few minutes it took to pack the things we needed to go out. One lot from whilst we were out (well two really but a damp bum from swings isn't going to do anyone any harm so was left). And then a small girl went puddle dipping in the garden when we got home too.

It's been wet here in case you hadn't gathered.