The plan to break up the trip sounds good. We did something similar, but stopped at
Disneyland on the way back instead of on the way to Hawaii (flying from home in Toronto so the time change was a whopper!)
We spent 10 days at Aulani and honestly it didn’t feel long enough. However I think that’s the way to do it - leave something to look forward to. We stayed there as a base while we explored the island and alternated resort days / exploring days so really it was probably 5 full days at Aulani.
Hard to predict what kids like and with a 2-10 split the activities will differ. DD was 8 when we went, she loved: getting early breakfast at Off the Hook together and watching the sun come up, all the community hall activities, stand up paddle boarding, snorkeling at the resort, playing on the beach including the fun mermaid activity offered, star gazing, listening to stories around the fire, the pools (no adult pool at the time), and she absolutely loved the lazy river. She hated Aunty’s so we spent the whole trip together. The scavenger hunt was okay. She hasn’t been into character meets for years so we waved to them around the resort but no interactions otherwise.
Off site we did a luau (she hated), short hike to a waterfall (hated), scenic drives (hated), shell collecting at various beaches (loved), watching surfers at the North Shore (loved), trying different local restaurants (mixed, she didn’t care for shaved ice as much as us parents did and she hated all musubi including Mickey ones, but she loved malasadas), horseback riding (loved), tours (hated Pearl Harbour and just refused to go to the palace).
I’m probably forgetting a lot but with a 2 year old you may end up staying at the resort a fair amount. There’s definitely enough to do at Aulani for 6 days.
Edited to add: Also visited the Dole site, she hated all the activities offered and didn’t care for the food options, only part she loved was watching the cats there. So we had some beverages in the sun and watched the cats for an hour while DH explored. It was nice and relaxing. I’m making it sound like DD hated a lot of stuff we did, and it’s true that she never wants to go on a hike or to a luau again, but she absolutely wants to return to Hawaii and Aulani specifically. Like, now. I think going with the flow and letting everyone pick 1-2 activities worked out for us. Just wish there was more overlap on the preferred activities!