Hi, K.,
I'm a massage therapist and have worked in rehabilitation settings, and I have a couple of suggestions.
First of all, traditional sit-ups don't actually do much for your abs. They are actually more for your hip flexors - not something that will really show in your waist line.
There are a couple of great alternatives. One is called "blow outs." You can do them standing, sitting or lying down. You start by forcibly exhaling until you can't blow out any more. This works the transverse abdominals. You can hold it for increasing periods of time (10 seconds, 20, 30, etc.) for increasing numbers of reps. Once you are familiar with the sensation, you no longer have to blow the air out but just tighten the muscles.
Most people are familiar with crunches. You lie on your back, knees bent, hands gently supporting your head. You make a small "crunching" motion of bringing your ribs closer to your pelvis. This does not have to be a big motion to activate the rectus abdominus (the "six pack"), and it's better to do slow, small, deliberate motions than to do wild, fast or big motions here. Really focus directly on the muscles you want to build, and that will strengthen them faster.
If you had a C=section with either child, I would also recommend you get some bodywork for any adhesions/scar tissue. This will help you to regain abdominal muscle as you regain full range of movement.
Good luck!
L. Freeman
LMT