Those children will be born in a small room, surrounded by metal and aluminum, maybe a blow-up shelter, and will never once in their life leave those surroundings. They will never once breathe fresh air. They will live and die in an area of a total size of perhaps a small football field, divided into minimal rooms (so there'd be as many of them as possible), with minimal energy.
And that's just the beginning. From what we've seen happening to astronauts it is blatantly obvious that you do not want to subject a growing human body to gravity even slightly different from earth surface, to do otherwise will at the very least have serious consequences for their bones. It is likely in the extreme that it'll have extreme medical consequences amounting to serious physical handicaps, even if they are born on the surface of Mars. If they are born in zero gravity, it seems unlikely that they'll ever even see adulthood.
But if they survive, it'll be in a small compound. Metal and stone around them, all the time, everywhere, and not much of it. Accidents will be as lethal as they are on a submarine and therefore they'll require the iron discipline (including the rare to occasional decision to leave someone behind to drown/suffocate when the situation requires it). They will be locked into this situation by physics, with absolutely no way to fix it. Even if terraforming ever works, it will take centuries. Dozens of generations. They won't just be old when things change, they will never live long enough to see children that will set a foot outside. They will see DVDs, and will get the phenomenon we see in the third world. DVD players showing them the vast fjords of Norway. The immense flats in middle America. And let's be realistic here: spending a week in Death Valley will look like an incredibly attractive prospect for them, but even dying of exposure there will be utterly out of reach.
Let's face facts here: these people will have lives that solitary confined prisoners would not trade place with.
We are the ones, along with their parents, who will have "done this" to them. Their parents will be forgiven, as they have made a mistake, listening to the misrepresentations of people like Elon Musk. We, however, "could fix this", with incredibly massive energy expenditure to get a few thousand people back from another planet, and we'll choose not to. Why not ? It'll make the Moon landings look incredibly cheap, hell it'll make WWII look incredibly cheap, and even if we do it, it'll take decades for the transport to actually happen. Even if we choose to have a kid come back, the kid won't be back until (s)he's in their forties at least.
We are probably a decade, maybe 2 or 3 removed from having built "general AI" individuals, who would have no more trouble walking in robot bodies on Mars or Venus than we do on earth. As in, a bit of planning would be required, and you'd do well to have some basic preparations and shelter ready, but failing that will be unpleasant, not deadly, and it would take weeks of exposure and deprivation to kill them. Besides, such individuals would be programs walking around in robot bodies. If they really got into trouble they could just transmit themselves to safety.
Since these will easily colonize the solar system, and humans won't, not now, not ever, even if we manage to get a tiny Mars colony going, why not just wait ? Bots, AIs, cylons, machines, or whatever we choose to call these individuals will far outnumber people on Mars (Given the surface size, a Mars colony should expand to about 6 billion people, but it will take a little under 1500 years to get to that point, so even if we start with 1000 individuals (100 is more likely), machines will start to colonize Mars (and other planets) when there's less than 10k humans)