I was in the same boat as you when I first started. I would agree with Daniel's assessment of the situation, but would add a couple other things to the mix. I had the 70-300 and was thinking that the 300 was going to be the best choice of the bunch, but soon found that once you get out past 200mm with that lens, the sharpness begins to really take a hit. I found that for wildlife it was a little short, but then again...I only see a deer every now and then. Birds it was way too short. I sold that lens and purchased the 70-200 f/4 IS when I had enough money saved up. That was one of the best decisions I made. I think that the 1.6 crop factor in your t3i will give you more than enough reach if your chasing after your kids. It does work for wildlife but you will do some cropping. I used my 70-200 with a kenko 1.4x tc for the longest time and had great results. My selection would be with the L and if you had the chance, get the L with the IS. You could look for some used on KEH.com or check the inventory at the Canon refurbished. You can sometimes catch a special where you get 15% off there. I know Adorama and B&H have a used and refurbished department also. That would be my suggestion.