You love the 50mm f/1.4. My assumption is that you like it because it wasn't too expensive, it's very fast at f/1.4, and it's not too bulky or heavy. You want to be able to freeze motion because kids don't sit still, and that's only possible at shutter speeds speedier than 1/100. And you have to ask yourself, how much weight do you want hanging around your neck if you're simply playing with your daughter at a friend's house. Do you plan on expanding your arsenal even more when she starts crawling and walking? And do you ever plan to upgrade to a full-frame camera? If so, do you mind having lenses designed for cropped sensors?

If you want relatively inexpensive, small, light, but still very fast lens, I would say pick up another prime lens. f/2 or faster. I'm assuming that you find the 50mm too telephoto for your uses. The Canon 28mm f/1.8 or Sigma 30mm f/1.4 will do just fine. The Canon 28mm f/1.8 is slower than the Sigma but the Canon 28mm will work on both EF and EF-S bodies. The Sigma will work only on EF-S bodies but is a full 2/3rds stop faster.

The 24-70mm f/2.8 zoom is also a fine lens, but it will be bigger, bulkier, and more intrusive (due to the tube design), as well as very expensive. No zoom lens will ever be as fast as your 50mm f/1.4. The 70-200mm f/2.8 is a great lens but giant and heavy. It costs an arm and a leg. I wouldn't carry one around unless I had to. I've found that for indoor/low-light shots, I need that extra stop I can get on a prime lens.

The flash suggestion is good too, but only if you have the time and energy to learn how to use it. Would you keep the flash on your camera at all times, so you can capture something the second your darling does something cute? If not, then stick with a fast prime for available light shots for now and save your money for future gear expansion.