We also set the rule that if someone falls (because shaving cream does get slippery) that everyone stop spraying until they make sure the fallen person is okay.
Good thing we have those handy goggles (you can also find goggles at the dollar store this time of year, but I don't know how reliable they are).
Then, we simply turned them lose and were prepared to watch the entertainment. Nothing like watching your kids make a big mess in the yard.
There were so many laughs and giggles. What is more fun than making a huge mess?
And when they couldn't see? We just hosed them down gently with water. They knew not to try to take the goggles off.
Once they were all done, they all got a thorough hosing, and then played in the yard until they dried out a bit.