For this you wouldn't use an app on the ipad. It needs to be something like lacing cards or beading a shoe lace, basically anything that your child has to grasp/use their fingers with.

Dexteria Jr.-Fine Motor Skills Development App. Our OT adviced us it's really helpful.

As someone with Cerebral Palsy, I use an ordinary writing pen for fine motor skills. Three intervals of five repetitions each. With each repetition, I must close and open my fist before picking up the pen and placing it back down. Above all, I repeat this exercise three times a day.