I actually have two recommendation of great apps with loveable little monsters that your kids will love:
One app that goes free today is Monstory - A memory game with fun tones that plays each time a kid flip a card. There's no difficulty levels, but the set of cards somewhat changes on each game. Overall, a small brain trainer for children at the age of 3 to 5.
Usually this app cost $99, so if you miss this free offer, here's a monster-alternative app called Poke Me! HD. Now, I don't know why I haven't tried this before, but surprisingly it is one of the pet apps that is really free (from ads also). Your kids will enjoy to play with, feed, and poke this cute purple monster.