As many have said, the 17-55 is a great choice. Can't go wrong with it, and it's useful in so many ways.
A big question is whether you expect to use a flash or not. I know you said you're looking into the 430exII, which I also own. But it's not always at the ready.
My experience is that f2.8 is often not wide enough for indoor shots without a flash unless you have a 5dIII or 1 series body that can really push the ISO. That's why I really like fast primes for indoor shots of kids (I have 2, aged 4 and 6). I think the 35L is the holy grail here. Alternatively, for far less money, the Sigma 30 f1.4 has served me pretty well. If money is not a limiting factor, go for the 35L.
FWIW, I found the 50mm focal length on a crop sensor like your t4i to be too long for many indoor uses.
Good luck!