1) I don't think Tiananmen square takes that much walking. Go in one entrance, look around, and go out another, at the forbidden city (north) end.

2) The forbidden city is a lot of walking if you want to see it all. It is very large and there are numerous corridors and buildings to visit. You may not want to see it all, but even a little bit is a good amount. Not saying they can't do it, but I was quite tired walking around it for 3 hours or so.

There are bathrooms in the FC at various points, but not all conveniently located, IIRC.