Hi Tehani,
Jim B alerted me to your request for information about the PG. We've spent 24 days on the PG and loved every minute of it. As was mentioned above, be sure to get water shoes for both your son and yourselves. Unlike the Caribbean there are few long sandy beaches in FP. You need foot protection from broken pieces of coral, the rare, but potentially fatal, cone shell and sea urchins to mention a few. (Don't let these cautions scare you - we were in the water snorkeling once or twice almost every day.) Also try to take Dominique's guided snorkel lesson and tour on Motu Mahana. Even though my husband and I have snorkeled extensively during the past 35 years we learned a couple of new things from Dominique. He is a great guy.
I have my doubts about you being able to arrange a dive for two adults with supervised snorkeling for your son but it won't hurt to ask. There are certain cruises where they have some programs for children and if a dive excursion happened to fall within the same time period it might work.
You will not find long rows of taxi's, seashells, tour vendors etc. on any of the islands but you'll find some shops and some rental cars particularly on Moorea and Bora Bora. There will be some local vendors even on Motu Mahana and there are several shops right at the dock on Raiatea. But whichever location you are at on Sunday you will find the shops closed. On Moorea you can get a free shuttle ride to the pearl store that has a shop on the ship. It was called the Black Pearl Gem Company when we were there but I've read that it is now named for the designer and co-owner Tahia Collins. That shop is within walking distance of several others in an area known as Petite Village (unless they've also changed locations). You just tell your driver which shuttle time you want to take back to the ship and you'll have time to check some other shops. I seem to remember a seashell shop there.
If you dock at Cook's Bay on Moorea there are several vendors under a large tent nearby and I think they had shells in addition to stuff like a shark's tooth on a leather cord necklace.
I think it was on Bora Bora that I picked up a flyer at the dock to walk across the street to a pearl shop and receive a gift. The gift was a nice oyster shell. So keep your eyes open for little things like this.
If your cruise is one that spends two days at Bora Bora you will be closer to town the first day and closer to the other private motu on the second day. That motu has a long sandy beach and your son could enjoy the sand and also snorkeling. However the snorkeling there is in deep water so it would depend on how comfortable he is with that. Be sure to take plenty of waterproof sunscreen and a sun protection shirt that he could keep on while in the water would be best. My husband and I wore "skins" on the days that we spent more than an hour in the water. They covered us from neck to ankle.
When are you going on your trip? Ask away with more questions. We'll all try to help.