This is disgusting and I need help

Okay so my son had a sippy cup that's been missing for a week (that was full of chocolate milk) and I finally found it. Hidden under the mattress thing in his play pen with the milk actually leaking out of the cup in the play pen.🤢 How do I clean this up? I have to use it like right now or I would just soak it in the tub. There's milk all in the bottom of it and it stinks so bad.😭