If you’re transiting those airports and want better facilities for baby changes and the like, even in some lounges, then KL or DL via Amsterdam. With Amsterdam you also avoid the security rescreening hassle, and that makes a difference when you’re fully loaded with a baby. AF is more of a stickler with cabin bags at times than KL, but in practice that varies by station.

I would suggest DL or KL via AMS, whichever one goes from SEA most directly to where you want to go in Europe.

lap child infant fares are 10% of an adult fare. If there is a seated child discount it’s probably no longer more than 25% off the adult fare, if even that, for AF/KL/DL on SEA-departing routes, but I could be wrong about that. For such a long flight, you’ll want either a bassinet for the baby or a seat in which to use a car seat to secure the baby. Bassinets aren’t always available on flights as other passengers may get or book them first. But the only way to guarantee a place for the baby to rest without having to hold it all the time is to buy a seat for the baby and have an approved car seat with you in the plane cabin.

Bassinet seats are always the bulkhead row IME, and you may get them with just a lap child ticket. In economy class I dislike bulkhead seat because the leg space doesn’t work for me and the because that is where I most likely would encounter immovable armrests that make the seats feel extraordinarily tight even compared to the other economy class seats.

It’s sort of like “pick your poison”, the poison being the kind of discomfort and expense you’re willing to endure whichever choice you make.

Thank you for the information Guwonder. I too prefer AMS for transfers but forgot about the going through security again. That’s a big difference. Do you have any idea which aircraft offer the most legroom and best bassinet setup - DL767, DL330-300, KL777?