Check my setup in the signature, I 75% of images are of kids in the younger than 5 years..
I love the 24, 35 for laying on the floor when they play. Outside i also use those two for the most. The 50 gets a lot of use always, and equal the 85. I can't really see me using anything else... the closer the better. I don't like a 300 to "not interfere" and shoot from a distance, because first off, you often loose the connection to the subject, and second, you isolate the subject, whilst I found setting and surroundings where they like to play much more interesting. If you see a wooden wall behind the kid you fail to see his favorite climbing rack you would've gotten with the 24. Why i love the lenses I have is because when including the background with wide lenses, you can make them less intrusive with a much larger aperture.
Only my opinion..
I agree with Neuro about the 85 for portraits of course, and in a birthday setting or something is very different from out playing with one or two kids. I like to take my son to kindergarten on Sundays, then we can shoot on all of his favorite climbers and slides with on clutter and interruption.. i'll post a few samples later