Either have him ready at the door and hand him out, or bring him to mom, communicate whatever you need to and then say "I need to get back down to the other kids, so I'll see you tomorrow" and walk away. If she lets her kid go and he comes back downstairs, then you escort him back and sternly tell mum "you can't let him run all over my house, when it's time to go it's time to go! Thanks for your help."