I dived Turtle Reef in April and it seemed pretty healthy to me. There has been some deterioration in the soft corals and sponges over the years, but no more so than the rest of the island. Vis was 50-70 at the time.
There was still lots to see, particularly as a night dive. No evidence of excess dolphin and turtle droppings.
Shore diving at Turtle Reef and Lighthouse is relatively inexpensive (10-12 a tank) compared to boat diving, and is a pretty nice way to spend an afternoon or evening.
There is a good bar near the entry at Turtle Reef to hang out at after diving, as well.
I don't think that having a new operator at Cobalt can change the diving. From shore there in the summer, I would guess that you will be able to dive most days. The only downside is that if you are staying and diving at Cobalt, shore diving at Lighthouse will not be included in the package price.
LCBR usually offers some summer deals. I would check them out. IMO, LC beats GC by a mile as far as diving is concerned. They also have bikes, kayaks and a hobie for rent at reasonable price, so plenty for a family to do. Not much near Cobalt Coast.
Also, have a look at the resorts on the east end of GC, they offer nice beaches, snorkelling, watersports as well as some of the best diving on GC.