If you can stand it, I would wait until you are done with children. Tummy tuck is a procedure you only want to have done once!
If you can't stand it, go ahead and get fixed up. I have had several patients have a surprise baby after a tummy tuck and their pregnancies went fine and their abdomens actually snapped back pretty well. But you need to be prepared for a revision or even a full redo.
As for the hernia, I commonly repair these at the time of tummy tuck. I slit the belly button down the middle from the center to the 6:00 position, find the hole in the fascia and stitch it up with a few stitches and sew the belly button back up. I'm super careful with the blood supply to the stalk of the belly button and haven't lost one yet (knock on wood).
I know some plastic surgeons like to have a general surgeon help with hernias but I did full training (6 really long years!) in general surgery prior to doing specialty training in plastic surgery so hernias don't scare me.
Aren't children the best? I would have gone for number 4 but I was too old!