Having a tummy tuck is going to make pregnancy more uncomfortable and potentially risky. Plus, a pregnancy will likely ruin the results of your tummy tuck. Re-doing it will lead to additional scarring since you will have extra skin, but not enough to completely redo the procedure. Don't do it.
Once you are done having children, wait 6-12 months so your body has time to return to normal first. Pregnancy causes a lot of hormones to be released that make your connective tissue stretchy, and you want to wait until that effect is gone. Your final result will be better for it.