goal - one sit up

Find an abdominal workout plan and do it 2-3 times per week. P90X has an awesome one. Don't get discouraged if you can't manage to do even half of it at first. Do what you can and push yourself. Even 1 week later you'll be impressed how much more of the workout you'll be able to do. You'll be doing old-school situps before you know it!

Eat less & move a little more

I was in the exact same boat, could only do crunches! I have been working out for about 3 months, and this whole time myself I could only do crunches. Then today I was doing a boxing fitness class, and the coach said situps...and I did them. Not only did I do them, I did a 100 of them total! They may not have been pretty but I did them, finally overcame what my brain told me. Set small goals, 5, 10, 20...build up to it

This totally happened to me! (or I tried the same thing, however you want to phrase it).
Step one: Have your child hold your feet and do one. You actually can but its a bit of wiggle
Step two: Do step one for 5 days
Step three: Do one on your own for a week
Step four: Do 5 on your your own for a week
And repeat adding 1 each week. Sounds ridiculous, I know, but this is how I did it. I only got up to 20 and it took a while.
Next is push ups:)

This is a great plan. If you have no one to hold your feet, you could lodge them under something like the edge of a bed.

If you find that uncomfortable there is another variation: hold on to a dumbbell or similar weight with both hands, lie down on your back with straight legs and with your arms stretched on the ground at the same level as the rest of your body above your head. Think superman position, but on your back (but make sure to have a very good mat under you as set ups with straight legs could be taxing for your back). Lift your arms off the ground and towards the front and do a setup. The weight will pull your body forward making it much easier to perform a setup.

Now, once you are comfortable, try bending your knees like the classic set up dictates. Next step would be to simply use smaller and smaller weights every time you feel you can handle more until you can do it unassisted with free arms. After that, you could advance to the "hands on shoulders" kind.