Is it possible to play outside at the end of the day? Then when mom comes to pick up you say have a good evening, get your kids and go inside? Or another thing you could do is have your kids stay downstairs (depending on age of course) then bring the child to his mom and then go back downstairs. Surely mom wouldn't allow the child to run around your house if you go downstairs. Definitely a gate will help. Also explain to the child that when mom comes he needs to go home and that running around your house is not allowed. Some people just don't understand boundaries!!