I am so sorry

but he is 5 years old and should not have been unattended for a minute, but it's NOT your fault, I think this child needs to be talked to immediately and told that that what he did was the wrong thing to do. Did you speak to the parents, did you speak to him? I have done the same thing many times, watched from an open door and nothing happened we all have done this so it's not your fault. I pray your little baby will be ok

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A 5 year old does not have the full understanding of what he did, he probably knows it was wrong from everyone's reaction, but he doesn't understand the severity of what he did. If you don't feel comfortable with watching anymore then I wouldn't, unless your going to be hands on with him when you are watching him. The child definitely needs to be talked to and I hope he was.
It takes 30 seconds for things to go wrong, I know I have been there.
prayers and thoughts for your little baby and you.