The medics do things to them to enable them and their descendants to resist every possible condition of temperature and radiation and gravity. They have to alter the genes—acquired characters would be of use only in a short-term project, and this is long-term. But you can't alter genes without affecting the individual.
We'd have Hal's normal child.
But when Hal got to Lydna, he and the rest of them would be shocked and sick for a while at sight of some of the inhabitants. And if he had any children on Lydna, we, back here, would scarcely recognize them as human. Some of them might have extra limbs. Some might have eyes and ears in odd places. Some might have lungs outside their bodies, or brains without a skull.
By that time, Hal himself would have got over being sick—unless, some time, he got hold of a mirror and remembered the boy he used to be.