That's a difficult question. Most of the AI's in St. Lucia are couples only. The beaches/snorkeling is also not that great. It's more about the topography and sightseeing.

Here's a few AI resort's that you can check out and see if they meet your criteria:

St. Lucian by Rex Resorts

St. James's Club, Morgan Bay

Calabash Cove Resort & Spa

Sugar Beach, a Viceroy Resort

Coconut Bay Beach Resort & Spa

Smugglers Cove Resort & Spa

I have no idea which is the "best" AI, do you mean best price or best location or best food?

Most of the hotels in St Lucia offer meal plans and that may include an AI option. The only ones that have compulsory AI and are couples only is Sandals and probably few more others, eg Rendezvous. Some of the more upscale will only offer up to MAP meal plan. Jade Mountain does not accommodate children under 15 & also does not have AI meal plan. None of the hotels mentioned above are for adults only.

Coconut Bay is on the Atlantic side so the beach is not going to be as calm as on the Caribbean side.

Anse Chastanet has a beautiful beach and a marine reserve, you can go AI or not at this hotel. There are kids there. Their website says they have 2 beaches but I have only seen 1 of them.

http://www.ansechastanet.com/activities/activities.html

Windjammer is on a lovely light(er) sand beach and has various meal plans inc AI. It's not a couples only hotel.

Bay Gardens gets good reviews, I would not expect any snorkelling on Reduit Beach, but the beach itself is very nice, lighter sand and clear water.

I'm not sure what people mean by the beaches and snorkelling are not great, maybe because the sand is darker in St Lucia, at least in the jungly south of the island. The north has lighter sand if thats your preference. But the snorkelling is quite good, at Anse Chastanet marine reserve and Anse Cochon for example (opposite end to Ti Kaye).

Compared to other Caribbean Islands such as Turks & Cacios, Grand Cayman, St. John and even Cozumel, Mexico and some areas of the Riveria Maya of Mexico, St. Lucia does not have great snorkeling or beaches. The draw there is more about sightseeing.

If the OP will provide more specific information about what they are looking for in a vacation besides AI's and snorkeling, we can probably provide better suggestions as St. Lucia really isn't the destination if that is their main goal.