I had this problem when we used to look after my nephew, starting around age 10. He had no problems going alone, it was just more of a safety worry for me, given there had been a few nasty incidents reported in the media around that time involving boys of a similar age. So I would *always* stand outside making sure to announce that fact ("I'll just wait here then") and believe me I would have no qualms about barging in if I suspected something was wrong. Also helpful if you get them to tell you when it's going to be a longer toilet trip (so you know when to worry), usually they're just in an out quickly.