is it a saltwater tank? unless its a huge tank i would just clean it myself like every week or so, to be honest i dont clean my tank as much as i should but it seems to be ok.
at first i cleaned it every sunday, but then i got lazy and i only clean it when its really dirty.
since your daughter is only 7 i wouldn't advise letting her clean it alone, but it could be like a family thing, u know, it could be fun. and it could also be a learning experience
YOU EAT SHARK=SHARK EAT YOU!