Excellent post Benjismom. You saved me from writing an extra long post, as I would have recommended the exact same things
I would add in doing Dead bugs
for the transverse abdominus, the deepest layer of abs. They work as your built-in girdle.
I would suggest that you do modified versions of the front and side planks first.
To modify the front plank, place your elbows on the floor and clasp your hands. The base of support will be your forearms on the ground, intead of your hands. Also, push up from your knees (like in a girlie push-up). Contract your abs and keep the body in a straight line from your head to your knees. Do 3 sets. When you can hold this position for 1 minute, progress to the next level; pushup from your toes instead of your knees. The front plank targets mostly the multifidus muscles which are in your lower back and act as the back half of your built-in girdle. You can work your way to doing the front planks and variations as described my benjismom.
To modify side planks, start off by bending one or both of your knees to the floor. Again, Do 3 sets and hold as long as possible, and working your way up to one minute. When you can hold the modified version for one minute, progess to the next level as described by Benjismom.