FirstFable. Not only that, but all the current character packs are available in this Bundle, which is also free for download. This is part of what is aparently 'Teach your Kid to Game Week'. As addictions go, this is one that I will not be too displeased for the sproglets to get hooked on.
The characters currently available are Pirate, Knight and Fairy Princess. I'm not sure how I feel about the fairy Princess as a character (although I know it will appeal to LP), but I haven't read the rules,and of course there's nothing to stop the girls picking the Pirate or Knight, that's just how RPGs work.
Another set of rules out there for kids is Hero Kids. This one is not free for download (but the colouring book is). Apparently, the main difference between the two is that FirstFable requires a lengthy character creation process (although I assume lengthy here is relative, and we're not talking Travelleresque), whereas Hero Kids comes with pregen characters. I haven't gamed with my kids (in terms of RPGs), but I can see that maybe some kids would like to get into what their character is all about, whereas for some they might to want to get stuck right in.