+1 on recommending the 17-55 f/2.8 or a fast prime, and the 50 f/1.8 is a great ($100) place to start, as it is an inexpensive way to see what fast primes can do, but the AF is slow, so for capturing pictures of fast moving children, you will likely want something with a faster AF.


Others to consider, 85 f/1.8 (check out Baker's current thread on Basketball), EFS 60 f/2.8 macro, 50 f/1.4 or the 35 f/2. I only own the 50 f/1.8, but have considered buying all of those lenses at one point or another. I would look at what focal length you currently use the most with your 18-55 lens and either buy a prime that has that focal length/works around your house (if you are always at 55 mm, one of the longer focal lengths may work) or buy a faster zoom lens, such as the 17-55 f/2.8.


Good luck