I recently let my 6 yr. old twins brush their own teeth in the morning. But... I still brush their teeth before bedtime. I have been told that kids don't really brush well until at least 7 to 8 yrs. old. They may look like they are doing a great job by themselves, but I don't think kids get way in the back all that well. Therefore, I make sure to give them each a great brushing before bedime! I'd rather take a few minutes to do it myself instead of having to deal with cavities. And... so far, so good... no cavities yet!
And as far as washing hair goes... my daughter likes to do almost everything by herself, including washing her hair. So we alternate... she will wash her hair and then the next time ( usually 2 days later on), we wash it together. (She does some lathering and then I continue to lather and I do the rinsing.) On her own days, if I don't think she rinsed all that well, I will tell her to rinse again. (She gets sensitive about this sometimes... but, oh well... if she wants to do it on her own, she needs to understand that she may still need some coaching.)