A simple search of “apps for praying the Rosary” turns up about 540,000 hits of one sort or another, but the one I’m familiar with is free and very easy to use and has a simple interactive element that may amuse your children while they pray.  Laudate has tons of basic Catholic stuff (I always use it to follow the readings at Mass), and one of the things is a simple Rosary that keep count for you as you tap the next bead in the picture.

Not a lot of bells and whistles, but if your child is entertained by an iPad but you don’t want a lot of other distractions, this may be a good fit for you.  (And the price is right.)