My son and I sometimes wait, think result/weather etc can depend on the response.
Choudhury, Albrighton, Barnes, Perez and Ricardo were particularly good with the kids. Perez actually ignored his security guy and came over the barrier so that he could have a photo with a girl in a wheelchair.
Albrighton and Choudhury seem to go around the crowds and see most people. Albrighton is good at away games too.
Last week at Newcastle Iheanacho came over to my son after he heard him shouting him! We had the most joy away at Cambridge in pre-season, these games seem easier to get to them, I guess due to the relaxed nature of pre-season and the lack of security at those games.
I would say they start coming out approx 1 hour after end of the game at home, earlier at away games. Depending on the stadium it can be easier to get them there.
Hope this helps!