hey hey
i have had mine done since i was 12. i did it myself with a needle. when i was in 9th grade, my best friends dog ripped it out... painful. It took forever for it to be re-piercable because of the thickness of the scar tissue. i got it done again when i was 19. Had a piercer do it that time.
... Anyway, i have never had a problem with mine. But the main reason i am writing is because now i do piercings and have spent the last four years in a tattoo shop that does piercings, so i know a little about it....
Belly buttons take the longest to heal of all piercings. On average, total healing time is 4-6 months. And you can be almost garaunteed it will get slightly infected in the beginning. Nothing serious or to worry about, just a little uncomfortable, and another reason to keep it clean. Contrary to popular belief, the best way is NOT hydrogen peroxide. From a piercers stand point, hydrogen peroxide should only be used on earlobes. It's not harmful to the belly button, just not recommended. The best thing for all piercings is Dial anti bacterial soap. Any time i get a new piercing i have a little bottle i keep in my purse. Anytime it starts to hurt or itch, just go wash it with Dial and it should take care of it. Mine got infected when i got it done at 19, and i needed a little more than just Dial. (i don't think i went to a very reputable shop, but i was still young and uninformed). At that point the best thing to do is soak the piercing in water with sea salt. The sea salt will draw the infection out. It's gross, but it works.
As far as getting pregnant later, as long as you take it out prior to your belly getting big, it shouldn't leave a tremendous scar. If you leave it in through the pregnancy, it will. But if you remove it when you find out you're pregnant, it shouldn't scar, other than a little dot. It will probably close up. (People heal differently. i could leave mine out for years and a ring would still go through.) If it does though, it is re-doable... and it usually doesn't take 5 years for it to be so, unless of course a dog rips your skin off while tearing the ring out. (i just got lucky).
Your girls should be just fine. And, it's better than them asking for a tattoo, because piercings are never permanent. Just make sure you do research on shops in the area to find one that won't cause permanent damage.
i'm biased, though, i have 17 piercings all over the place...