Last time my MIL babysits!

I left my 20 month old with his grandparents . When i returned to pick him up both my husbands parents and the Only adults in the home were sitting outside on the front porch ... my son inside with a 6 and 3 year old alone on the top of the basement steps ! The 3 year old with her hands wrapped around my sons ankle trying to pull him down and the 6 year old losing grip of his hand trying to hang onto him! And the baby in absolute panic ! The 3 year old was not supposed to even be around my son as she is aggressive and has severe behavioral issues as well as was removed from preschool for being physical! And she left my child with this little girl with no supervision. Not only was he put in a very dangerous situation, but then I get him home to find a bite mark and some goopy crap with glitter all over his private area. I am furious. There is no way I could ever trust my son there again and they all think I am rotten for no longer allowing her to watch him. What parent would send their child back there? Just needed to vent. Having a hard time letting go of this one.