Kapalua and Wailea are about 40 miles apart. And, granted, that distance can make a difference in Hawaiian weather. But let's not confuse Kapalua with Hana.You get more clouds in Kapalua than in Wailea but we're not looking at dramatically different weather IMO. Brief rain showers are common in Kapalua where you'll find the weather mostly dry in Wailea. But you're not going to see the Hana rain-forest weather in Kapalua.
I spend a lot of time in the Kapalua area in the winter months and find the weather just fine. And the bay will be just fine for a 7 year old. Kapalua also isn't the North Shore like Paia...the water is fairly calm in Kapalua Bay in the hotel area.
I stay in the Lahaina area myself on our frequent trips to Maui as we like the variety of restaurants and activities in this area of the island.
Enjoy your trip!