We're hoping to do the exact same thing, at least on our two MK days. My evening ADR's are in the 4:30-5 range, so we plan to be on the first bus to the park, do as much as we can by 11:30, eat lunch there (and grab some extra FP's before and after). Then bus back to Pop so we can rest in the room, change shoes, jump in the pool, etc. I have just my DS16, but he truly melts in the heat. I'm thinking if we leave the parks by 12:30, get back to Pop (hopefully) by 1, have at least 2 hours to rest and swim, and then start getting ready for the trek back. If it doesn't work out for the first day, the advantage is that we can just stay the second day for the afternoon (and use our FP's and do more shows and cooler stuff) and then just call it an early night after dinner.