What time of year are you planning to travel? That makes a big difference in regards to price and water temperatures. Florida has a lot of good options but the water is cold in the winter and spring IMO and the same is true for the Bahamas.
Many dive ops in tropical dive destinations offer "Discover Scuba" dives that allow you to try scuba - first in a pool and then during a shallow ocean dive with close supervision by a divemaster. The rental gear is usually included in the price of the Discover Scuba course, see the link below for an example:
https://www.sunsethouse.com/grand-cayman-try-scuba-diving
Trying it out first with a Discover Scuba class might just be enough to scratch their itch - or it could help your parents decide that they really want to go all the way and get certified. You might be able to do a Discover Scuba class fairly cheaply over a long weekend in Florida; just be aware that you need to allow 18 to 24 hours of non-diving before your return flight after diving, to reduce chances of decompression sickness.
If they decide that they want to go for Open Water Certification then I suggest you make arrangements with a local dive shop to do the course work and pool work in advance, and then do the check-out dives on vacation. They should also buy their own mask, fins, and snorkel in advance and get them properly fitted at the local dive shop. Nothing ruins the experience quicker than a leaky mask, and they can always use this gear for snorkeling whether or not they decide to get scuba certification.
Four divers getting certified at the same time might be a bit much for a dive shop to handle, while they are also serving other customers and handling normal operations, so be sure to make the arrangements in advance.
Keep in mind that your parents are going to need passports and that can take time, so that should be their first priority - and perhaps it's another reason why Florida may be a good first step.
Others may disagree, but I'm a senior and I don't consider Mexico or Central America to be to be especially senior-friendly. You have to be careful about food and water because food-borne illnesses are not uncommon. When traveling to Roatan and the Honduran Bay Islands, travel medicine physicians advise getting Typhoid and Hepatitis A and B vaccinations before you leave the US. They also advise taking anti-malarial medications before, during, and after the trip - and that all adds expense to the cost of the vacation. Plus you have to keep applying insect repellent because the mosquitos are voracious and Dengue and Chikungunya are endemic. We've visited and dived in these regions when we were younger but now that we are older we look for places a little healthier and easier, but that still provide good diving and a lovely tropical vacation experience.
Nowadays we prefer the Cayman Islands because they are clean, safe, and healthy and the diving is very good. It's easy to get there and English is the official language which might be pleasant for your parents, since this will be their first time traveling outside the US - it could give them a chance to sort of ease into the experience.
The Cayman Islands are not generally considered a cheap destination, but it can be cheaper and easier to fly there as compared to some other destinations, and you might be able to find specials and reasonably priced options - especially if you travel during the lower seasons like summer and fall.
We like the Clearly Cayman resorts because they are small, diver-dedicated, and all-inclusive resorts (food, accommodations, and diving) see the link below.
https://www.clearlycayman.com/
Clearly Cayman has 3 resorts total with 1 on each of the islands (Grand Cayman, Little Cayman, and Cayman Brac.) Reef Divers is the onsite dive op for all of the Clearly Cayman resorts and they provide valet dive services. That means that the dive staff does all the heavy work. They carry and set up the gear, help you into and out of the gear before and after the dive, and switch your gear over between dives - unless you tell them that you prefer to do it yourself.
I am sure that the new divers will need to demonstrate that they can set up their gear before they can be certified, but after that they can let others do the heavy hauling. Scuba gear is heavy and you might get stuck doing all the carrying.
The CC properties are nice, clean, and friendly resorts with good food. I would suggest the Cayman Brac Beach Resort as a very friendly property for seniors (and other divers of various ages and abilities) although any of the CC resorts would be fine IMO.
The cost of certification and gear rental is likely to add to the overall price wherever you travel since you each hope to do a total 10 - 15 dives. When you are comparing prices across different locations and venues, make sure that you understand what's included. Sometimes there are significant increases for taxes, resort fees, airport transfers, single occupancy, etc. The prices quoted for the Clearly Cayman resorts usually include everything but the dive shop tips. There may be an extra charge if you want to add-on a beverage package. We usually bring some alcohol with us into Cayman duty-free.
I hope you find a good option that will work for you and your family!
