My DD is 7 and has dark skin and dark hair. Her hair is already dark on her legs and I am just waiting for the day she comes home and says someone said something about it. I will be taking her to have laser hair removal. I have done them all for 30 years - waxing, shaving, electrolysi, laser, depilatory creams and laser is the way to go. I did not find that laser was great for my upper lip but I don't think I went to the right person a she just used the same "wand" or whatever they call it as she did on my legs. I think you need something a bit smaller to put on the top lip to put it on the right angle to really get all the fine hairs so definitely make sure you go to someone who knows what they are doing.
I think waxing is too painful for an 8 year old but I found laser less painful so I think it would be OK.