My understanding is that Moffat suggested (not asked) that Jenny should survive simply because the audience would assume she would die, so it would be more surprising.

Ah! That's a shame. NLSS Child would certainly have liked to see her again. The character would have needed some work to give it a little more depth, but there was nothing really wrong with it in this episode beyond being a bit one note.

What a strange idea to commission an episode to demonstrate the Doctor is capable of change and not build that into an arc of some sort. Obviously arc building over multiple writers is non-trivial (and I have a vague memory that The Unicorn and the Wasp was filmed first so maybe they intended it to air earlier in the season) but to do it in some kind of arc-free way just seems a bit potty... and come to think of it, if Silence in the Library/Forest of the Dead had followed straight on from this then you would have two stories in a row in which first the Doctor and then Donna have to come to terms with the loss of children which might have worked better, although given this season isn't really building up to anything particularly about family in the way it is building up to something about soldiers, I suspect it would still have seemed a little awkward.