The Ponyo song also had a little girl in the song, maybe they topped also the digital chart. i don't know
I know Gake no Ue no Ponyo peaked at #1 on RIAJ's monthly chaku-uta chart. I'm not sure about the weekly. I know it got 500,000+ certification for chaku-uta and 250,000+ certification for chaku-uta full. However, Ohashi Nozomi was about 8 y-o when the song was released, much older than the now 6 y-o Mana-chan and Fuku-chan. In term of age, I think these two little adorbs are the youngest so far.