Can you do regular crunches? If your gym has one of those ab-roller contraptions, try doing that for a week or more, and then try the incline again. Or, if you can lower the decline so it's not so steep (closer to flat) that might be a good place to start.
I know when I had gallbladder & tubal ligation surgery, they do cut through the abdominals, and it felt like I'd done too many sit-ups when the muscles were healing. Since a C-section's a much bigger cut, I imagine that would be more difficult, and the amount of scar tissue would make you a bit "stiff" unless you losened them up and build them with some crunches. So, maybe your muscles ARE weak. Don't be afraid of ab-work though. Just start smaller - the incline is actually a more advanced movement. Do some crunches and bicycles. Bicycles are quite difficult (while lying flat) and will help your lower abs, which are typically weak, under-developed, and overlooked by most people, because it's a hard area to exercise, and they think that crunches work the whole muscle. (Crunches generally only work the upper rectus abdominus.)
As for your daughter - I was in LOUSY shape as a kid, but I could still hang from the monkey-bars by the knees and curl myself up repeatedly. Kids not only weigh less, but they're constantly using their muscles, unlike us sedentary adults.
Hope that helps!
--Janis