Congratulations, I'd definitely be thrilled to score any family beach cabana like that!
I don't think the walk is a problem at all if you just set your expectations. We had #5 last time. When a few of us walked along the beach to go buy a milkshake, all the kids wanted one when we got back (of course!). That did feel a bit like "oh no, I have to take that walk again" (and I usually run 6+ miles every other day).
To be very honest, that's why I won't list any cabana higher than #6 among the priorities when I submit concierge requests for our next cruise in a few weeks (even if some of the higher #s have slightly better views, more privacy, or other pluses).
However, the privacy and quietness of the cabanas and along family beach _are_ really nice and I will be happy with any number that I might be able to get. Especially now that there are rest rooms closer to the higher numbers. If you walk along the paved road, the walk to the restaurant isn't a problem at all: it's just long enough that I'd ask _all_ the kids whether they want a milkshake before I go
Also, if I had #20, I'd set my family's expectations that they might want to walk the short distance to Family Beach. The small pond near the higher # cabanas wasn't as appetizing when we were there in November: the sand felt more grainy and the water wasn't as nice. None of this is any issue at all, except perhaps if everyone arrives with sky high expectations to everything.