We were pretty OK with our flight back in 1995. I believe we flew into Frankfurt & out of Heathrow, but am not really sure (some airport in Germany in & some airport in London out). The long 12+ hour layover in Newark was exhausting, but at least we were able to rest in the red carpet room that trip.
I would love to read about your experiences. H is leaning toward just going to US national parks and NOT Europe. We will have to see what the kids have to say & their schedules.