ive met plenty of parents who have come to a compromise similar to this nature in an effort to help their child learn and grow in a less volatile environment. if i was going to pick an ideal i would say personal apartments next door so they get to learn how to look after themselves and a house, bills, meals and all in-sundry before ever contemplating moving in with another person.
but… most are far from the opportunity for “ideal” in some forms of philosophy(emotional, experiential or financial).
I hope it goes well for them what ever they decide and i fail to see any advantage (statistically or otherwise) why a young couple should rush into marriage before at least living together for a few years.
what i would define as ideal would be dating for 3 years wit no major issues then living together for 2 years with no major issues then marriage if that is what is important to them after around 6 years, thus creating an emotionally mature, safe and stable(financially included) environment to have a child.