I have 5d Mark III, Tamron 28-75 2.8, and few primes 40 2.8, 50 1.4, 85 1.8, 135 2.0, and finally 17-40 that I got recently (used) and seem to rarely use. I'm mostly taking pictures indoors, fast moving twins, trying mostly without flash, as I can't bounce effectively when subject is very close (jumping on me).
I use 28-75 quite a bit, always on 2.8, unless one of the primes is on (love shallow depth of field). I find that I often want just a bit more than 75mm to not change position and capture moment.
So I find myself constantly drawn to 24-105, though I rented it when I had 5D mark II, and I wasn't crazy about it, 4.0 felt slow inside of my house. But, I feel like it would be such practical lens because of range, set and forget, that I'm thinking of selling 17-40 and buying 24-105. But then, I can spend few more bucks and get Tamron 24-70 2.8. Currently, in Toronto, it would be less than 50% to jump from craigslist 24-105 to retail Tamron 24-70 2.8.
Am I just going crazy because I want to spend some money? Or is there real benefit to getting one of these 2...
with the 5Dmk3 the absolute best lens for shooting your kids in what you describe is the 40mm f2.8 which you have
its very light and short so easy to shoot one handed even indoors your 5dmk3 is good at high iso to keep shutter at 1/80 or 1/100 sec its vey sharp at f2.8 it focuses accurately and it has a 250mm minimum focus distance so very easy to shoot if the kids are right on top of you.
Any of the zooms with the longer barrels are going to be alot more cumbersome to engage with the kids especially if youa re playing with them as you pretty much have to use both hands with the 5D3 and 40mm is very easy to shoot 1 handed