How old are your nieces?

10 and 12, but the last one is a little behind her age (and having the short attention span).
Also got a 5 yrear old nephew, but dont think he would be up for it yet.

Hero Kids is a very good RPG for young kids. In fact, just two days back I ran a game for my nephews and they really enjoyed it. You don't even need any prep work - just print out the essentials, give your kids the character sheets, have them fill in their names, and you're good to go.

Edit: not in btw, just wanted to chip in my 2c about the game

The game's rules are so light you can just about make up your own rules on the fly. If you can tell a good story, I think your 5-yo nephew will enjoy it ;)

The 5 year old will probably make his own rules on the fly. Playing Basement O Rats with my 5 year old was nothing like the planned adventure. Still, it was interesting, and it turned out there were sharks in the pool under the basement and there was a giant scorpion in that part of the cave, but it was good and it went up and ate at the tavern.

I'll play it with my 14 year old brother

Whoa! Interesting... Okay. I'm in for this giveaway. Thank you!

Anyone else?

The random gods have spoken, and the winner is Treasure. Congratulations!