Simply not true. Does your mouth "stretch" and stay big if you have bitten into a big sandwich, altering your appearance and the working order of your mouth? No. Does your anus stretch "out of shape" and stay that way if you are constipated and have a huge poop? NO. Do your nostrils "stretch" if you sneeze a lot or pick them all the time? NO.
Vaginas are similar. They are made to stretch during childbirth and go back to the same size. ALL women have the sides of their vaginas touching while they are at rest. Normal childbirth doesn't ruin vaginal muscles.
All women should probably do PC muscle exercises, to keep good pelvic health. This is whether they have had children or not.