I highly recommend going in March so you can see the whales off Maui. You'll see them right from the bank, but going on a whale watch on a sailboat is awesome.
There are two main tourist areas on Maui: Wailea and Ka'anapali. Wailea is like the Beverly Hills of Hawaii. Ka'anapali is older and attracts more of a middle class. Personally, I prefer Ka'anapali as it has more to do, and most importantly, I like being close to Lahaina harbor. We are on the water quite a bit. That is where a lot of whale watching tours launch from.
Most privately rented condos (e.g. from VRBO.com) have minimum stay requirements during high season, which would be March and June. April and May are low season. However, others can be rented from resort management firms for just 3 nights. I own a timeshare at the Westin Ka'anapali Ocean Resort Villas, and they're great for families, especially the south side. (Though I'd recommend booking at least on ocean view here because some of the island views overlook the parking lot and highway.) The condos at Honua Kai are pretty new and beautiful. But both of these resorts are north of Ka'anapali Beach, which is tourist central.
If you want to be in the thick of action, then I'd opt for Ka'anapli Alii or Marriott Maui Villas. In Wailea, I'd check out the Polo Beach Club, though the pool isn't nearly as fun as you'll find at the huge hotels such as the Fairmont or Grand Wailea.
Hotels such as the Hyatt, Westin, Four Seasons, and Marriott have great kids' programs. The Hyatt also has a rooftop astronomy program that your son might enjoy.
If you belong to Costco, look to them for deals on hotels and car rentals. They usually give complimentary upgrades or hotel credit. Honestly, you can't go wrong on Maui.