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simple fix: No kids on mars till there's air and grass.
I've done some calculations, and it should only take ten years to build a suitably breathable atmosphere on mars. It is entirely conceivable that one could truck in several hundred tons of water Ice, then electrolyse it into atmospheric oxygen, and truck in some nitrogen or extract it from regolith, producing a breatheable, albeit low pressure atmosphere within 2 years, and under greenhouse conditions, the whole planet could concievably warm up enough to release the remainder of its adsorbed carbon dioxide within a 10 year period. Mike Zubrin makes the claim that something on the order of 20 to 50 years would be required to get an atmosphere going just to support humans walking around (still with oxygen masks) at something like 8 or 10 psi. (in his book "The Case for Mars")
I think you could do it much faster than that, and without breaking anyone's budget. Also, if you wanted to quickly get some terraforming happening, simply take several "burn barrels" of dense hydrocarbon fuel and some form of oxidizer, burn them at the north or south poles to create a soot plume which would settle on the ice cap, and you instantly vaporize the polar caps with solar radiation. That's what happened in greenland a while back. A large volcano erupted, spewed dark ash over a large piece of ice sheet, and then the ice sheet melted under the dark ash, producing a lake on top of the ice, heated by extra solar radiation trapped by the soot. Then there was a flood when the water broke free, but on mars the flood would be CO2 gas, which would leave the soot where it was, on the surface, and it would simply fall onto subsequent layers of carbon dioxide ice.
With a quicker terraforming campaign, the idea of raising kids on mars becomes quite a bit more feasible, since you don't have to stay inside a hab 24/7.
Hey, they already used your idea in a ramjet engine. Kaiser Marquardt, the big 1950's ramjet researcher built a solid rocket motor inside a ramjet, and there was an ablative nozzle on the engine. When the engine burned, it changed the shape of the nozzle, since the rocket would be going faster and faster, and eventually switch to burning kerosene as a ramjet. The nozzle went from a very large aspect ratio to a rather small one, since the flow through the engine would already be close to supersonic, the ramjet nozzle only had to restrict airflow a little to make the jump to M>1 flow, then expand it out to a mach number of M=2 or 3. The slope of the nozzle wall would be shallower, in other words, than the rocket's nozzle. Older designs used a small rocket motor completely ejected from the ramjet prior to mode change, while this engine used the remnants of the rocket motor to auto-ignite the kerosene fuel for the ramjet. I believe development continued into the '90s, resulting in an operational muniton for the RAF and several other european air forces. I'll see about a link for you later.
You have an interesting idea in applying the ablative nozzle for altitude compensation, however. Try getting a book, Introduction to Space Technology. It's got everything from rocket motor design to orbital mechanics. Barnes and noble 30 buks I believe. Hasta Tarde,
Rion Motley