Best learning apps for kids?

I bought my toddler a tablet for Chrismtas and am needing the best learning toys for him…what are your kids favorite apps?

Anything leapfrog, ABC mouse, and PBS stuff

I let my 4 yo great grandson watch Evan Storm and the axel show,( if he likes monster trucks) for an hour.after he does his school work that his parents and I have prepared for him.