I did a lot of travelling on the tube with my kids and always said that if we get separated, get off at the next stop and wait on a bench , I will find you or if you are on the platform wait on a bench and I will come back. It was a sort of rule so we all knew what to do. I always made sure they were in right in front of me at all times , eg not to the side of me getting jostled about, which seem to help, getting on and off. I still say wait at the next station now to anybody that I’m travelling with !