Thank you everyone, the advice given here is awesome! To answer a question, she got a new 5D Mark III so she was going to sell the 5D anyway, then I saw her at a wedding, we haven't chatted in awhile but we used to be super close, she was in my wedding and we lived together in college, and she said she'd give it to me for 350. She's amazing and very generous.
I had decided to get the 5D from her and then realized that it has no internal flash, and ironically I JUST sold my speedlite because I never used it. I love taking pictures of my kids with the camera but not carrying around the gear, since I usually have a baby on my hip and a diaper bag in hand while chasing a 4 year old. I decided it was too heavy and bulky and I wasn't going to learn how to use it properly. So now I'm trying to think of how often I use the internal flash on my T1i, because with the 5D I would have no flash option at all. I haven't decided if that's a deal breaker!
The "built in" flash of a rebel is limited to max 1/200 max flash sync. The flash really isn't all that useful anyways since its not capable of H-sync (ettl high speed sync). Your better off using an external flash most of the time.
If you buy a 430exii flash it'll be beneficial for both your current T1i and your potential 5d full frame. If your going to document your kids...try to maximize the best image quality as possible. Kids grow sooooo fast!!
I struggle sometimes in lugging my youngest girl and handle my 2 other girls on an outing. This is where I'll use my panasonic gh3 (killer HD video and "good" image quality) or I'll use my Olympus OMD Em-5. The olympus has excellent image quality in a very small package. Thing is I have much more control using a 5dc or 5dmk2 for any type of photography. The micro 4/3 small bodies will do a great job (image quality easily on par or even surpass your T1i).
I do find that I can use my spider holster and lug my dslr's no problem with the girls. I just get moods where I'll lug little tiny camera with big image quality.
5dc isn't much of a learning curve. It's no more difficult than any canon dslr body. If anything it'll be easier.
IMO one of the best bang for your buck crop bodies with great IQ is the Canon 70D. This is not going to help your issue of not having a wider angle lens. As for full frame the 6d but has a 1/4000 max shutter speed so may have issues in outdoor bright lights with an f/2.8 lens. 5d classic is 1/8000 max shutter speed just like the 5dmk2 and 5d3 which gives you some more flexibility.
When I owned a crop camera I bought the best glass I can afford to maximize the best IQ I can get out of a crop sensor. NOW the zinger is if you heavily invest in an awesome 17-55 crop lens ......will you stick with a crop body? Or just buy a very capable tamron 17-50 (non VC version)....then later go full frame??
I'd jump for the 5dc and invest in full frame compatible lenses.
Dont make a flash an obstacle. YOU NEED an external flash regardless of crop or full frame ownership.
If you buy the latest small micro 4/3 mirrorless camera its more expensive than a Canon 70D. M43 are great small camera's for no flash good light photography. You can also buy a small lens/flash setup too. Whats the catch? Perfect for form factor but a new system $$$$$$. New m43 systems are better at low light high iso performance than the 5d. Image quality in the 5d simply rocks and is fantastic for family documentation compared to the latest crop camera from canon.
I think you should start a well discussed topic of "should I go full frame or stick with crop" and mention your criteria of current lens ownership.
Tell us your budget.....