I think something to look for is locations that have smaller diveops. It sounds kind of selfish, but what you want to do is ensure that the 4 of you are the largest group on the boat so if/when the diveop allows you to pick/suggest locations you can pick shallow ones. And tell them upfront that you are so they can alert the other paying divers. Especially during spring break when you can expect everything to be crowded.
I'd also look for something a little removed from the mainstream during spring break which leaves out most of Mexico and anywhere there's significant cruise traffic like Puerto Rico or St. Thomas.
As an example, even though Grand Cayman typically does deep 1st dives on the Wall, a smaller diveop like Indigo or Ambassador is only going to take 6pax per trip. I believe Indigo only takes 4 on their smaller boat. And there is a lot of good shallow diving off Cayman. A couple of the "Gardens" sites south of town were only 30-40' as I recall.
In addition there are shoredive options that are optimal for your teens, you can hire a DM at most of them with notice. Turtle Reef is one of the best dives I've done there, it's 40-50' max to the sand, the good stuff on the wall starts at 20'. There's easy access via a ladder and Sundivers onsite for the DM. In town, Eden Rock/Devils Grotto doesn't exceed 40' afaik, a lot of it was 15' from the surface. And there's Stingray City, the most fun you/your teens will have diving. It's 14' deep sitting on the bottom. Downside with Cayman is that it's wicked expensive topside. You could also dive a lot of Sunset House's reef, they have a Saltwater Pool (open to the ocean) for entrance and a lot of shallow reef. The classic dive there though, the Amphitrite Mermaid is about 55' to the base.
The Virgin Islands might be another option, especially the BVI's. The smaller islands south of Virgin Gorda - Cooper, Norman, Salt, Ginger etc. all have great boat diving in very shallow water. Most people stay on Tortola or Virgin Gorda. One way to get there less expensively is to fly into St. Thomas and take one of the hourly ferries over. They leave from both Charlotte Amalie and Red Hook on the East End - getting to either costs about the same as it's a cab or a shared van to each location - google BVI ferries for a schedule. Leverick Bay is a somewhat reasonable option to stay there, they have a beach and diveshop onsite.
The Aquarium, one of few dives just off Virgin Gorda was 40'. Obviously lots of fish there. There's a bunch of sites off both Scrub and Cooper Island, at Scrub I don't think it's possible to get below 50'. Some of the Cooper dives were good at 20'. We did Alice's Backside at Ginger Island, not only was it flat calm but the whole dive was 20-40'. And interesting stuff to see, lots of rays IIRC.
DiveBVI is the larger diveop on VG, during the week we did a couple dives with a brand new young teen diver and his dad - they provided a DM just for them.
I might rule out Nassau as Stuart Cove's has a lot of boats and often a lot of divers per boat. They do however break their groups out by experience level. Bahama Divers, the other diveop, does the blue holes and more advanced diving.
There are some really shallow dives near SC's - Hollywood Bowl actually starts in 10' -the vast majority of sites they do are in the 50-70' range though. They have an interactive map on their website with details.
hth,
