Usually, if you're in a special situation (with infants, or with large luggage or a large instrument), you cannot use the automated check-in machines. I have had this issue flying with a Cello (that I bought a seat for) and with someone that required accommodations (pre booked). Also worth noting, you cannot pre-book seats selection if a leg of your flight is booked through another star alliance member, no matter how much you want to pay. The only way to *try* to get around this is by buying business class tickets and getting the airline to try to accommodate you with phone tag (i do not recommend) because they're just more helpful with business class. This is a pain in the ass with international flights because even though you may book a flight through Air Canada, or Lufthansa, the flight may be actually run by Swiss or any other Star Alliance member. And no, it makes no sense to book each leg with a different airline, you'll end up pay 5-10k for a flight.