Good Canon lens for kids sports photography

What is a good Canon lens for capturing kids sports? I'm talking 4-5 year old soccer & t-ball & such right now, but obviously more as kids get older. I currently have a Canon Rebel XTi.

I've borrowed the Canon 70-200 2.8L and it's a great lens but kinda heavy. Anything comparable without the bulk or weight that will make me actually want to carry it around on my camera and capture pics from the sidelines? My only lens right now is the 24-70 2.8L and I don't feel it's enough on the telephoto side.
Thanks for your responses.