I would tell the parents, but I would be more likely to send an email as to not bring it up in front of the child. My child is almost 6 and he'll have the odd "didn't make it to the bathroom" moment. He cleans himself up and brings the damaged clothes to the laundry room and tells me.

I would let the parents know that he was concerned about bringing the dirty ones home so you washed them for them. You don't want to mention it in front of him as it seemed like a really sensitive subject to him and no need to upset him further. It was just that... an accident.

At the end of the day, we are not the parents, we are the caregivers. A 4 year old doesn't have rights to privacy. You're commitment is to the parents and it's your responsibility to let them know.

Can you imagion if you didn't tell the parents and the little boy let it slip? They it looks like you're keeping secrets! That's not good.