Demon Child

Have you ever been friends with someone who has a demon child? That's a little harsh to say about a child, you might say while looking down your nose at me.
No, it is not, I submit and here is why . . .
I have a friend, a good person who is willing to do anything in the world for you. She is the kindest person, a good mom to 3 kids and the most unpretentious person you will ever meet. I adore this girl. Her demon child; however, not so much.
This kid is cute until you've been around him for about 10 seconds!
He has one volume- loud!
He runs around screaming wildly. (ALL OF THE TIME)
He hits.
He spits.
He is mean to other kids.
He interrupts every conversation that his mother has in his presence.
He does not lisen to adults/authority figures. (blatantly ignores)
He is too rough- he hurts himself and other children daily.
He gets this mean look in his eyes when he knows he is doing something to hurt another child. (I watched him throwing those sticky little gumballs, from trees, at other kids. I told him to stop. He glared at me and immediately began trowing them again. By the way, his mom was not there and I was in charge of him- I was not just randomly yelling at other people's kids that day.)
I can't stand this kid!
Last summer, he tackled my 3 year old in the baby pool and was lying on top of him holding my child under water! My child who can't swim! They invited my child to the pool last week (I was not able to go) and the mom said he could just go with them. Not only NO, but H-E- double hockey sticks-NO! I had to make up excuses as to why he was not able to go with them.
Just avoid the kid, right. Wrong. It is hard to do when he and my son go to Pre-K together and the mom and I are such good friends, plus we work together. The kids play sports together, we workout together. Endless time with said demon.
The problem arises when my friend offers to watch my child. I used to take her up on it, but my child would come home behaving like a neanderthal. I now try to avoid the interaction altogether. Tit for tat, if she keeps my child for a little while, then I am going to have to keep her demon child when she asks. It has happened several times now and I just cannot do it again.
I am avoiding getting the kids together right now, but I know that the time is coming that I will not be able to avoid it anymore.
How do I handle this situation? No mother wants to hear that her child is horrible. No friend wants to hear that another friend thinks that either. I have kept this to myself because the friendship is important to me. Let's face it, we mother hens will tear somebody up over our kids.