This thread is timely. My 12 y.o. niece showed us her bikini this weekend and my MIL flipped out because it was so age-inappropriate for her (extremely busty child). MIL is looking to take her to get a more modest suit and I was suggesting many of these same stores. We are going on a family cruise together soon. My SIL got mad and said that there are ONLY skimpy suits available. I agree it's harder to find some coverage, but it's very possible. I have two teenage DD's myself so I have walked that minefield many times.
I had to find a one piece for DD 17 for a summer program at Notre Dame (it was required). I didn't think a Junior size one piece existed, but I found a cute modest one at Penney's.
DD wore her suit from last year to a swimming birthday party this year and OMG!!!! What looked cute last year looks extremely suggestive this year.
That said, my DD is not EXTREMELY busty, but she is definitely well developed for her age. PacSun was a good fit for her so don't completely rule it out.