The originally published version Podkayne of Mars has its ending edited from being grim ?
I wouldnt want to go near books where they changed the ending....
Sorry, I wasn't clear there. The original (published) ending left it ambiguous whether or not Poddy would survive; in Heinlein's original, she will recover. The revision cut out Uncle Tom's speech to her father, which repeats something Heinlein had said many times elsewhere; and it also shortened the bit about Clark's interaction with the "fairy" cub.
However, the grimness comes in where Clark is concerned. Uncle Tom's warning to their parents, and the fact that Clark simply doesn't get what he is talking about, is one of the darkest possible endings I can think of. There is a faint ray of hope that Poddy's self-sacrifice may, just possibly, turn the tide... but it is very, very doubtful. And so what you may be dealing with is a very smart, very savvy, sociopath. Heinlein leaves that one open to question, but with the very strong impression that, if Clark is to be "saved", it will be touch-and-go.
I think the problem with this (for most people) is that the original ending has a bit less impact on the heartstrings, especially when it comes to Podkayne herself. As it was originally published, what has happened to her has a very painful emotional impact and leaves a lump in the throat. In Heinlein's original, the focus shifts to Clark, who is the narrator of the postlude (in both versions); you are relieved to find Poddy will be all right (eventually), but then the alteration to Clark turns things into a much darker vein than they have (obviously) been up to that point. You can feel sorry for Podkayne in the first published version, but even if she dies, she goes out with nobility and kindness. She may be a bit of a Pollyanna, but she also represents the best tendencies of human beings to give of themselves. In the original Heinlein version, Clark represents just the opposite, and all the little things (which have, up to this point, seemed somewhat satirically or even cynically humorous) suddenly come home as a ghastly reversal of this idea.
I can see where, it being published with the thought of it as a juvenile, such an ending would be deemed out of place; but I think each ending has its strengths. As far as sheer artistry, though, I think Heinlein had it right, and the publishers had it right as far as popularity was concerned. Both have a fair amount of impact, but the first published version was more immediately emotionally impactful; while the ending as originally written has a subtler, delayed -- and quite possibly horrific -- impact instead.
By the way, you can find the novel as originally published, along with Heinlein's original ending, in a Baen edition first published in August 1993 (pb edition in June 1995).