Traveling with infants I wouldn't even consider Roatan. There's Malaria, Dengue Fever, lots of Mosquitoes, Sand Flies, Sand Fleas on the beach etc. Most people use Deet or some other product continuously to keep from getting bitten up. Sometimes it actually works. Also it's an offshore island in a third world nation that has the highest murder rate in the world. Not much of a hospital, many locals use Sister Mary's Clinic. We bring her bandages/aspirin etc. when we visit so they're not exactly state of the art medical facilities. Roatan seemed pretty safe but not exactly where I'd want to take my kids...
Bonaire is an option but the hospital facilities there again are marginal if needed. Also outside of the resorts none of the shore dive sites have any sort of facilities - most don't even have shade. So the babies/sitters would be sitting in the sun while the other couple dove. And most don't have much of a beach either. Or I suppose you could trade off driving/diving at the shore dive sites - most are within about a 15min. drive of town. There at least you could "dive the resorts" part of the time also. Bonaire also has Mosquitoes and Dengue Fever is a possibility. So maybe not the best for the kids either.
Curacao would be better for that. Same diving as Bonaire, but most shore dive sites have on-site operators, tank rentals, snack bars/restaurants or some sort of shaded benches etc. Also many of the best beaches are also the best dive sites. The only downside is that some of them are a farther drive - it typically takes about an hour to drive the length of the island. The better diving is out west or via boat east - most of the civilization is in Willemstad.
It's also easier/cheaper to get to than Bonaire typically - a lot of the flights to Bonaire actually stop there first. You can find pretty decent accommodations reasonably also. We stayed in a 2BR/2BA condo at Piscadera Bay Resort. Across the street from the Marriott and Hilton but where the Marriott was $400/nt. the PBR is $150. It's also a mile to a major market and there's some quick meal options - Pirates Bay beach bar, the outdoor restaurant at the Hilton or there's a Denny's just down the street. About a 10min. drive to everything downtown from there. You don't want to stay downtown, there's no nearby diving and a lot of congestion since the cruise ships moor there also. There's also the Sunscape AI south of town, it's close to diving and very kid friendy - a former Breezes Family resort. They may even have babysitting services there.
They also claim to have the best hospital in the Southern Caribbean there if needed - we were there one night, it seemed like a decent facility.
Curacao Travel- Caribbean, Curacao, Dutch Caribbean for everything.
Some areas of Curacao are a little lower-class though, there's both a deepwater port and oil refinery near downtown and some theft issues. You won't notice it at the resorts but almost all of them in town are gated. PBR has 24hr. gated security with a night watchman. So did the others we saw.
Compare/contrast the dive site differences here:
Scuba Shore Diving Region: ABC Islands
For shore diving you might consider St. Croix also. They have good shore diving off Cane Bay Wall on the north side and there's some condos for rent there on vrbo.com. Also a couple of dive resorts and other hotels/resorts etc. Frederikstad Pier is another option for shore diving. It is a little pricier there but it's also the U.S. They also have a world class medical facility should there be a problem. AA flies direct there from some U.S. cities also IIRC.
Grand Cayman might be another option but it's a lot pricier. In fact it's probably one of the most expensive dive locations there is. Being the offshore banking capital of the Caribbean means a developed infrastructure, lots of condo options on Seven Mile Beach. Boat dive operators will pick you up there daily and none of the shore dives are more than 15-20 mins. drive away - or less. Also all of them have a developed resort with food options on-site. And a dive shop for gear rental.
I could easily see taking the kids to a site like Turtle Reef and having one couple dive while the other watched them. There's both a Tiki Bar and an upscale restaurant with A/C and shaded outdoor tables there. That way both could get a couple dives per morning in. There's also the Turtle Farm nearby but at 4 months, they probably wouldn't get much out of that yet. It's also a little sad as it's a working farm. And the Butterfly Gardens. Sunset House is another option, there's a decent shore dive, covered bar, indoor restaurant etc. onsite. Just south of downtown.
Downside to Cayman is that everything is really expensive - the CI$ is .80 to the US$ and food is very high - in restaurants or markets. You can save some by cooking in your condo and some of the US chains (Subway, Burger King, Dominos, a Hard Rock Cafe) are there. To dine on the beach in an upscale restaurant can easily be $50pp. or more though. Accommodations are equally pricey but some of the condos along south SMB are reasonable if shared. Many are 2-3BR and more set up for family use. Again vrbo.com is your best source.
Cayman also has two world class medical facilities - all that tax revenue gets put to good use. And it's safe, crime isn't allowed there...lol.