Yes, Xanadu would fit your desires nicely. With their 2 & 3 bedroom units, you could space the family as you prefer. Beautiful seaside pool, free bikes, kayaks, new & well built units, full kitchen, great owners who live on the property, small & quiet with only 15-ish minute beachwalk to town. Plenty of restaurants and bars within easy walking distance. Susan in the office can book your trips and make recommendations.
Snorkeling: Alfonse Graniel, L'il Alfonse under "Goods & Services" menu above, phone: 614-5450; 226-3537; great guy & guide, wonderful with people & fish; ask him about other snorkeling sites after you visit Hol Chan & Shark Ray Alley.
Fishing: Ramon or Severo Guerrero, phone 226-2325, 226-2324; E-mail: email@example.com; great fishermen and fun to spend a day with; they cook a great beach picnic with your catch and will send fillets back with you for a restaurant to cook for future meals; they guide all types of fishing. If you are interested in a particular type of fishing they don't specialize in, they will recommend you to a good guide specializing in that area.
Sailing: Geraldo Badillo, 226-2264, fun and serene; you could book your group with him as his only guests that day.
Mainland tours: have had great experiences with Tanisha and Seaduced. Both have trips to several different mainland sites and wildlife tours. Seaduced used to lead kayak excursions around AC.
Local tour: Bacalar Chico with Excalibur Tours. You go around the entire island, stopping at several sites with snorkeling at the north end, which is pristine and uncrowded.
July and August are warm with temps up to 94-ish, generally with lighter breezes - great for water activities.
Happy family trip. There may be summer specials, especially if travel figures are down.