I just wanted to add reassurance. I had achy legs and knees all growing up and now my 3 year old has them too and has had it since she was about 1 and a half and I was told that she was too young to have them by my pedi and then a year later we got a different pedi and he said she certainly could be having them at that age, its rarer but still happens. I actually still get some pains in my legs when I am over tired and my dad does too so maybe its not necessarily growing pains, but just when you get too tired. My daughter will spend a good 3 or 4 hours fussing at night and we usually end up giving her Tylenol to help her sleep because it usually only happens like once a month.
If it gets a lot worse and she starts having other symptoms along with it, definitely get to the pedi though.