It depends on the kid and the situation. For many, not coming home in the middle of a foreign exchange experience is a good thing. Some programs organize travel opportunities during vacation periods, or the kid can explore Europe alone or with friends. It can be very interesting to experience holidays in a somewhat different culture and enjoy all of the local traditions.

If you're not sure, don't buy the second ticket as not taking the first USA to Europe flight will mean that the entire ticket is forfeited on most cheap fares. You can throw away the return portion of a RT ticket pretty easily, even though officially the airline would have the right to reprice it as a one way ticket.