Ponyo was, indeed, great for my kids when they were quite young. Kiki’s Delivery Service (perhaps the American name of Madame Kiki?) seems appropriate for young kids, too, but I was surprised mine didn’t get totally into it. For middle grade kids, Howl’s Moving Castle is amazing, as are Spirited Away and Nausicaa of the Valley of the Wind.
I haven’t seen Princess Mononoke in years; I should probably introduce my kids to that one, too.