Best Ipad Games for 4 y.o. (long term hospitalizations)

I know I know, controversial subject. Prior to our kid getting sick (FU cancer) I was not an ipad for kids person. But I also didn't expect my kid to be undergoing chemo or that we'd have to be giving him shots at home. We want to distract him during hard processes because his medical trauma is already through the roof and we've barely dipped a toe into the pool that is his treatment.

So, I'm looking for the BEST, most zombie-inducing ipad games and shows for a 4 year old. We want him to be his utmost distracted. He enjoys shows like Octonauts, Paw Patrol, Wild Kratts, and nature documentaries... He's into equipment, volcanoes and loves patterns. Please help a desperate parent out!