Adult settlers will start colonization, but eventually children will be born on Mars, the next generation of a growing population of Mars colonists. Either the progeny of the first settlers continue the colonization process or, to avoid inbreeding depression, embryos with a maximum of genetic diversity are brought up (inspired by The Cascade). If Mars born children are capable of returning to Earth, the Martian environment will need to be sufficiently interesting to keep them there.
Effects of gravity on growth
Experiments in zero gravity show that animal embryos do not develop properly in zero-g (reference?). Mars has 0.38 gravity, so, even if the said child was not deformed, that child would develop and grow in 0.38 gravity. So, the child may be perfectly healthy by Mars standards, but severely weak by Earth standards.
Children growing up on Mars will not come in to contact with the same microbes as Earth children would. Thus, it will be important to vaccinate them to boost their immune systems. If the Mars settlement has a continuous influx of immigrants, then the isolation problem may not be very severe. If the Mars settlement has extensive food production facilities, these may serve as reservoirs for bacteria that will enhance the children's immune system. Plants and animals living throughout the settlement would probably also improve children's immune system response.