Let her work out herself what she wants to play when she's old enough. Meantime if she wants to join in, give her a couple of pans and a wooden spoon to bash them with - a perfectly useful drum kit for now. If you start her on an actual instrument too young she may find it a chore as she gets older. Kids are just amazing at telling you what they want to do (like my daughter, who'll play every style BUT folk!).

I met a kid who was an amazing sax player even at age 11. He went on to win the Stan Getz trophy at age 17. He'd started on piano at about 5 and didn't enjoy it. His parents had the good sense to let him give it up. He chose the instrument he REALLY wanted within a few years and was playing semi-pro by the time he was 10.