I think i'm going to go for the 50mm 1.8 for indoor shots of the kids, is it useful for portraits too?
Whats a good indoor sports, telephoto lens?
I wonder if a good general outdoor telephoto lens would be better...
Yes it's a nice walk-around lens.
No - without flash it won't do well indoors. For indoor stills you'll want to add an external flash. For indoor sports it will be next to useless. I wouldn't recommend counting on ISO 3200 for your indoor sports, so that means you'll be wanting a 2.0 lens for everything except maybe ice hockey (where you might be able to get by with a 70-200 2.8).
The 50mm 1.8 is good for about 10-15 FEET (not yards, feet) of coverage
The 85mm 1.8 is good for about 20-25 feet
The 100mm 2.0 for maybe 30 feet
The 135 2.0 is needed beyond that and is good for about 40 feet.