When they ask to play in the fountain, stream, puddle... let them play. Here are the magic words, my friend:
"You're welcome to get wet, as long as I don't hear any complaints about being cold afterward."
If you do end up having problems, quietly say, "Ohhh, this is so sad" and nothing more. Yes, I realize it is challenging not to lecture them at this point, but it will bring out nothing but anger.
The next time a get soaked opportunity arises, very kindly say, "That didn't quite work out when we were at [location]. Don't worry, we'll try it again next time." They key to all of this is remaining calm and being empathic.
Will you give this a try with me if an opportunity arises? Honestly, does it matter if their car seat gets a little wet? Look at the pure joy on her face:
Memories like this will definitely earn you "The Coolest Mom EVER" award for the day.
One quick note, if you are on a play date, be sure to check-in with the other child's parent before letting your little one do something crazy-cool. Nothing is more awkward than having a set of pouting children on the sidelines. Luckily for us, our play date said, "Go for it."
Let your children take advantage of warm afternoons, remember:
"We don’t stop playing because we grow old; we grow old because we stop playing." — George Bernard Shaw
Are you letting your child play? Will you practice saying "yes" with me?