After age 6 or so it becomes more problematic, and I would not put myself in such a situation. Go with your spouse whenever possible. Little boys DO need to use restrooms, and moms certainly cannot go into a men’s room with an 8 year old son, nor should that son be in the ladies room.
Like oomis said, starting at age 6 it is a tznius prohibition for an opposite gender child to go into the other restroom. So will you leave a 6 year old outside by him or herself???