Dear 100 Hour Board,
What would happen if a baby was conceived, born, and raised in a low-gravity environment like the Moon or Mars? What would be the biological differences in a child in such an environment, and would they have trouble adapting to life on Earth if they ever visited?
A lot is unknown about adults in space, never mind developing children. In fact, to my knowledge (and consistent with a cursory Google search), no child has ever been sent to space. However, we can make some guesses.
The child would have a difficult time growing up in space. According to NASA, adults experience the following even with lots of exercise and healthy eating: high rate of loss of bone density, vision problems, increased isolation, changing microbes, and certain types of space radiation. There are probably other things too but let's just go with those.
The loss of bone density would be especially bad for kids. Bones respond to forces that are consistently applied to them by increasing ossification at the points of stress. This is why adult astronauts lose bone density. Newborns do not have much bone density to lose. Similarly, children would not have any gravity to work against to develop their muscles. I am not sure how gravity would affect growth, but at the very least we would expect children to be incredibly weak and fragile.
Vision problems, while problematic for adults, could be especially problematic for children. The ability of children to process visual information is dependent on the information that they receive. For example, children who have a lazy eye become blind in that eye because the information delivered by that eye is unhelpful.
Increased isolation... I am not sure how that would affect children because I'm not sure how big these lunar and Martian colonies are. They would, I assume, always be with decent parents, but if they grow up with only a few people around they might have a hard time adjusting to life on earth because of their limited socialization.
I am not really sure what would happen with the microbes.
Space radiation, which is higher than on earth even with shielding, can damage DNA and do things like cause increased cancer. So I am guessing the children would be at increased risk of cancer relative to the general (Earthling) child population.
Overall, I would recommend raising your children on Earth.