How old are the kids? Black Rock is a good snorkel but the swim out from the beach could tire little ones. If there's a current running, it's usually to the south so it helps going back. And they'd need some floatation probably. It's even a pretty good walk from the parking garage - unless you're staying at the Sheraton. The dive/snorkel site is the cove near "K" -
http://www.in2scubadivingmaui.com/images/MapsEtc/Black Rock Map directions.jpg
I like the suggestion of Old Airport also, it doesn't get very deep for quite a long way out and there's things of interest literally 20' off the beach. Big parking lot also - you'll find that although every beach on Maui is public access, finding parking at some is an ordeal after about 9AM.
Kapalua Bay up in Kapalua could be another option - nothing there is deeper than 40' and a whole lot of it is shallower. I've read Kapalua Beach is one of the nicest on Maui.
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands - this also suggests good snorkels.
Really just about every beach in the area has decent shallow snorkeling. Rule of thumb, if you see surfers at one, the viz/conditions will suffer. Another general observation is the better snorkeling (and diving) is often off the rockier points at either end of the bay.
Boat dives near Ka'anapali are done with Lahaina Divers, Extended Horizons or Hawaiian Rafting Adventures. There's one more also - always forget the name. Lahaina Divers dives both Lana'i and Molokini Crater on different days. They have two large Newton dive boats. The Cathedrals are the signature dive at Lana'i. They could have a lot of divers on the boat (as seen on their website) but they segment by experience level and it's 8divers/DM max on Maui.
If your dive count is accurate, I don't suggest the West Lana'i advanced drift dives, the currents can be strong. LD prefers you have AOW or recent Hawaii dive exerience for those trips. Both EH and HRA have smaller boats. They don't go over to Molokini since it's too rough coming back. All three are based in Lahaina harbor, a short drive from Ka'anapali.
Maui Dive Shops also have their own boat but they keep it in Ma'lea harbor - about 30mins. south of Lahaina. I don't see an advantage to getting basically the same experience that you will with LD with a longer drive. For tank/gear rental locally they're a good option - there's two shops in the area with rental tanks etc. Dive flags are required, they rent them. Or your dive guide will have one.
If you wanted to shore dive there's several guided options in that area also. Tiny Bubbles works out of the Ka'anapali Beach Club - depending on where you're staying (and how much gear you have) you could walk there. There's also a shuttle running around Ka'anapali, I never saw anyone bring dive gear on but plenty of snorkel bags. In2Scuba is based in Ka'anapali also - IDK where or much about them.
Scuba Dive Maui Ka'anapali Lahaina Hawaii Snorkel | 808.870.0878 | Tiny Bubbles Scuba Diving Specializing in Private Scuba Dives.Snorkel Tours. and Scuba Diving Instruction on Maui | Let's Get Tanked
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Your family can snorkel out on Molokini also.
Pride of Maui goes out there on a big powered catamaran from Ma'alea harbor. Kids 3-12 are welcome - where they moor it's very protected and shallow. Prices are a little high IMO but they grill lunch etc. Also it's easy entry, there's both a slide and stairs (not ladders) into/out of the water. You can even dive, they offer a dive at the same location also. I did it once a long time ago with an insta-buddy - we headed out deeper back towards Maui and found whitetips resting in about 60'. Cruising back over the coral we found a couple of frogfish also. I don't remember a DM but we would've not wanted one if they had offered. Someone qualified did do the briefing and led the Discover Divers though - we went the other way.
https://www.prideofmaui.com/
Another family snorkel option is with Trilogy Watersports - big sailing catamarans.
http://www.sailtrilogy.com/ Also out of Ma'alea. For practical reference it's a little over 30minutes drive from Ka'anapali to Ma'alea. I believe Trilogy has an 8AM departure from Ma'alea.