How common is it for young boys to show their penis to other children?

I was warned about this little boy (age 5) at the beginning of the summer by another parent on our street. He had shown his penis to another little boy and told him to “suck it”.

I was just informed by my daughter (age 6) that he had shown her his penis the other day when she was playing with him and his little sister (age 4) in their backyard.

I was furious. I never encountered anything like this when I was that young. My husband agreed with me that she is not allowed to play with him any longer, but then he mentioned that little boys do things like this.

I was shocked. My sons never did this, at least that I know of. Is this really that common?

How would you react if this happened to your child? btw- this boy also has a violent streak, he’s no sweetheart.