When I set out an in invitation like water and I do not want them to get soaked I just remind them "we are not in our bathing suits so water stays XYZ" insert in bucket or on the fence in this case. If they do not follow that directive they are done and sent somewhere else to play ... Kids learn quick if they want to engage in the activity they follow the rules.

With the squirt bottles today and washing toys they got damp by accident but not enough to need changing - I always have their extra clothes handy if clients wanted em changed.