We did our first split stay, also 10 days this past May at WL/Poly. I wouldn't worry about the ages of your kids, mine were 12 & 9 on our trip, and no issues at all, they were quite helpful with packing up, etc.
We planned to not be at the park on transfer day until late afternoon/evening, and had planned some outlet shopping or Disney Springs until we got our room at the Poly. We had a rental car so luggage transfer wasn't a concern.
On checkout morning, I went down real early to see if we could get a late checkout at WL, which wasn't possible, but instead they helped us get our room early at the Poly...at 9:30 am!!
Needless to say, right after checking into the Poly, we headed straight to MK,saving shopping for another time! It was awesome, so unexpected and something you could try, you never know!