Flights to BON are very expensive, though, so any other recommendations for a family-friendly Christmas trip?
I like Robint's idea; from another thread I know Scuba Club Cozumel is near Hotel Cozumel, and they have access to a shore dive right there. Which could be great for helping your kids get certified.
So, you're having to factor in price hauling a group around to exotic locales. I feel your pain. And Christmas tends to mean colder water in some locations; not the time of year I'd likely head to Key Largo (but I like water warm enough to skip wearing a wetsuit) - where I found about the cheapest boat diving I've seen yet (trip report - Rainbow Reef Dive Center).
Bonaire & Curacao are below the traditional hurricane belt. You've already been to Bonaire. I like Diver Steve's Curacao idea in part because it offers you a chance to keep costs down.
You've got to factor in total trip cost, not just airfare costs. Let's do a quick comparison to a mainstream boat diving destination.
1.) Airfare - both. Curacao tends to average lower airfare than Bonaire. And you're taking 5 people I think?
2.) Resort - both. At least in Bonaire, costs for adding extra people beyond the 1st 2 are often a good deal cheaper, since packages are designed to split a rental truck cost between 2.
3.) Rental vehicle - Curacao & Bonaire, yes. Other places - varies. But with youngsters, likely you'll get out and 'see the island.' And a vehicle can serve 2, or 5. No added cost for having the kids along.
4.) Boat diving - can be done on Curacao & Bonaire, but you can cram in a lot of shore diving for, on average, dirt cheap. Especially on Bonaire, but I would guess Curacao isn't much more (some sites may entail a small fee). Boat diving is all the diving at many places, most of it at some. On Curacao, you might do a couple of boat dives to expose the kids to it. Boat diving entails tips; shore diving doesn't. Tipping for a party of 5 at even $5/tank means $50 per 2-tank boat trip on top of trip cost!
5.) Number of Dives - Bonaire & live-aboards rock for banging out 20 - 25+ dives/week. Other's reports suggests Curacao trips tend to average fewer dives than Bonaire, but you can still do a bunch. And at this point, you can't know just how many your kids are going to want to do.
6.) Cost/Dive - a crude metric for evaluating trip value is Total Trip Cost/# Dives Done. So, do a lot of dives, average cheaper/dive. The truth is more nuanced; different places have different dive conditions. But it's not a bad rule of thumb.
7.) At some places, you'll need to pay in advance for a set number of trips/dives. Which means pressure to do those dives. If you pay for a boat dive package, what happens when 2 or 3 days into the week somebody doesn't want to dive today? The politically correct answer is anyone can call any dive any time for any reason. Decide for yourself if that's your idea of reality.
Christmas time, some smaller divers who would benefit from warmer water, aquarium-like tropical conditions for beginners, trip cost control via mainly shore diving to avoid boat dive expenses, no upfront paid commit for an expensive multi-day boat dive package, option for dive as a family & grow together where navigation is easy instead of following a guide around on boat dives, option for guided diving if you wish (e.g.: Bas Harts, Dive Bus), or do your own thing.
I've been to Bonaire 8 times. Only set foot on Curacao to change planes. But if I were juggling your priorities, and didn't want to hit Bonaire, I can't think of a better choice for you than Curacao.
About what airfare cost are you looking at?
Richard.