My dd is 7, so a bit older, but she plays with all the Angry Birds/Bad Piggies variants, Cut The Rope, Subway Surfers, Despicable Me, Mini Ninjas, My Little Pony, Temple Run, The Croods, Toca Hair Salon and Bejeweled at the moment.
We also have Squeebles Times Tables, Hangman and Duolingo.
Most of these are free/very cheap but lots have in-app purchases available, which I have turned off, so she can never be tempted to buy a massive basket of pony crystals, for example
I have a Nexus and i would like to add some apps for my DD 5 years, who has just finished reception. She enjoys music, reading, games etc and i would ideally like to add a range of fun apps that children of this age would enjoy.