yes you tip per person regardless of age.
When you think about it, if you went out to dinner with 3 people you would tip far more than $10.50 for the meal, so it just seems a lot because you pay in a lump sum
Our family has been on numerous cruises and we have always found our cabin attendants and dining room staff work twice as hard on entertaining our kids and keeping up after them than they do for adults. Although Celebrity suggests 1/2 of the adult tip for each child we always tipped them the same, or even a bit extra. A good cruise staff makes cruising with your children a wonderful experience.