It is a violation of UA’s Contract of Carriage to purchase a roundtrip ticket with the intention of using only one direction of travel — it’s called “throwaway ticketing.” However, UA understands that life happens, and they won’t penalize anyone for a one-off occurrence. Don’t make a habit of it, and you’ll be fine. That said, it sounds like you expect your child to make 1-2 trips home per year, so it doesn’t seem likely that this will happen.

You’ve mentioned purchasing a nested ticket starting from Europe, but as others have pointed out, that’s not really necessary — you don’t need a ticket for the whole school year with a nested ticket for spring break; you could book ticket 1, for “start of school until start of spring break” and then buy ticket 2, “end of spring break until end of semester,” later. That way, you shouldn’t have to worry about throwing any segments away, which really isn’t to your advantage anyway. (Even if it’s cheaper than a one-way in aggregate, keep in mind that you’re going to need a one-way ticket from Europe back to the US eventually).

The other pattern that might make sense is to use miles for your child’s outbound ticket, and then book all trips back to the US as round-trips from Europe up until graduation, by which point you will have had time to earn additional miles for the final trip home.