Here's the thing to know about Tempurpedics. They have to be "broken in." The ones you try out in the store, which are so comfy and inviting, have thoroughly been through the breaking-in process. Until they go through that, they're hard, like sitting on a block of wood hard. To break them in quickly, so they don't involve you sleeping on a hard wooden slab for several months, they recommend that you jump up and down on the mattress for a long time.
Even with that, the one I bought never became very soft. I returned it just inside the three-month return time frame. Three months of trying to make it bearably soft. (And then it took two years to get my guaranteed-no-problems money back.)
A lot of people return Tempurpedics for this reason, so there is a whole market of returned mattresses, which are sold by authorized dealers. They've all been returned within three months. One friend who did get one and kept it, got it cheap by this route.
But I'd say forget all that and buy a deep foam topper, or a cheapish regular mattress and a nice foam topper. Keep the queen-size. By the time your tiny ones are big enough to start crowding the bed, they won't want to sleep with you any more. And a king-size takes up a lot of room, in addition to needing all new items of the most expensive size of bedding.