I loved playing around with my 50/1.8 mk II for the shallow DOF, but it was poor at tracking kids because of the slow AF. Ditto for pets. It's certainly not the lens you want if you ever expect to be using AI Servo focus. The 50/1.4 is a huge improvement in that regard, as well as most others. I've heard wonderful things about the 30/1.4 Sigma, but unlike the aforementioned 50's, I haven't ever personally ever used that lens so I'll let others sing its praises. I do very much agree that 50mm is to long on a crop body for a general purpose indoor portrait lens. Something in the 28-35 range is more suitable.
If you've got a zoom lens like the 18-55, do the following test indoors during the day (when there is ample light): crank up your ISO so that you're not concerned about shutter speed, and just follow the kids around snapping the sort of pictures you intend to take, concerning yourself only with getting the composition right in the viewfinder (but not looking at the focal length setting on the lens). Then when you've got a bunch of sample shots, flip through them in whatever you use to look at thumbnails (Lightroom?) and check out the focal lengths that were used, as shown in the EXIF data. That will give you a good idea of what focal length lens(es) you should have in primes for this task.
You could also set the zoom to 28, 30, 35, etc.. and see if it works for you, but I personally prefer the "blind" test approach above, because it will reveal what you want, rather than what you could get by with. For me, I was all over the place and the results of doing the trial with my 18-55mm kit lens were that I used the whole range, and that's how I ended up with the 17-55 upgrade rather than a prime or two.