there are probably twice as many middle seats on a 3-3 narrow-body as there are on a 2-3-2 767
presuming you book window/aisle pairs in the same or adjacent rows of three, I would have zero expectation that *both* middles will remain empty
with that said, I’d book window/aisle doubles in two successive rows on the FI 763, and put each kid in a window seat (so they can lean against the sidewall) with a parent in the adjacent aisle seat