Both are potentially correct. It's also worth pointing out that you may hear American English speakers using the Present Perfect and Past Simple interchangeably in this context. But here's the distinction: You are right that the Present Perfect is the better construction to use here - when a parent says to a child "Have you brushed your teeth? and 'Have you washed your hands?', they are effectively asking "Are your hands/teeth clean now?". The Present Perfect is about actions - in this case very recent actions - which have a direct impact on the present moment. It isn't wrong to use the Past Simple, though. If you see your child coming out of the bathroom, it's perfectly correct to say "Did you wash your hands?". Here the implication is that you're talking about the (now finished) period when the the child was in the bathroom - hence the Past Simple. By saying "Did you wash your hands?" in this way, you mean "Did you wash your hands (when you were in the bathroom)?".

Have you washed your hands? = Are your hands clean now? Have you brushed your teeth? = Are your teeth clean now? You're correct that past simple tells us nothing about the present situation.