My daughter (5) loves to do this. I have lots of little and medium sized hair clips and also lots of barrettes (the more you have the better). So I gather them up and then she gets to brush my hair and put all the clips and barrettes wherever she sees fit. (Sometimes I have to adjust them because its either pulling my hair or poking me in the head) I look quite silly when she is done. But she loves it. Then, sometimes, its my turn and I do the same to her. Or I have a turn first that way I am forced to leave my hair done up. Then I polish her nails, fingers and toes.
I remember me and my older sister fixing my moms hair the same way when we were little. I have been told that I would pound the brush on her head in my efforts to brush her hair (unintentionally of course). Oops. Luckily my daughter is very gentle with me.