We ate a meal at Aulani (as adults) with Auntie! (we are suckers for all things Disney). We walked around the whole property and my opinion is that Aulani is much more low key and more appropriate for younger kids (like yours). While there is a zero entry sand beach and a lazy river, I think the Grand Wailea is more suited to (slightly) older kids because of the slides (big slides), Tarzan rope swing, and just the volume of people in the pools. I don't know what the lazy river was like at the Aulani, but at the GW you don't need tubes (most don't use them) and the current is pretty strong - no really swimming against it, if you miss where you were headed to you'll find yourself going around again. And truthfully I had to exit once at the zero sand beach and thought the sand was very rough, more like fine pebbles than sand. On the other hand, the sand at the GW beach was really soft and nice.

Again, this is just from "observing" Aulani, not from actually staying there or getting in the pools.