Was it really cold?
My son and his friends played out in the rain every chance they got if they were together. Some of my favorite memories are of all of them soaked through and having a blast. And it's not as if she sent 4 or 5 year olds out in the rain. I don't know what kind of school clothes your son wears, but unless they were dry-clean-only I bet they'll be fine once you wash and dry them. I hope so.
I could definitely be wrong, but unless it was REALLY cold, I don't think you can get sick from being outside in the rain, or from cooler temps, if that sets your mind at ease any.
To me it sounds like the kind of fun memories are made of.
