Most time you don't need ask. You can tell s/he is done or not by looking. If s/he carries all his/her stuff out, then s/he is done, otherwise not. Sometimes you can tell the game status by his/her expression, sad or happy. Of course, many kids have adopted poker face after years of tournament play.
You can offer your child food or water, but nothing more. If you do talk to each other, try to use English, not your native language, such as Russian, Hebrew, Hindi or Chinese. Mysterious language will arouse suspicion and cause some complaints, which will unnecessarily distract your child from his/her game.