US to Euro travel itinerary with two babies

So some people might think we are insane but my sister and I want to do a couple week euro trip in September when my baby will be a year and her baby will be about 2.5 months old. She and her husband get great parental leave benefits and live in Atlanta, GA so there are ample direct flights and with enough notice, myself and family can take off for a couple weeks at a time.

I proposed London for a week and then an Airbnb for a week in the English countryside to relax but she is proposing a road trip through the Dolomites. That seems really ambitious with two infants but what do more experienced travelers think? My husband and I would be leaving from SeaTac so there are far fewer direct flights to Europe than from Atlanta - I’m hesitant about the Dolomites because the trip between the US and Venice would be longer AND we would need to rent a six person car for both sets of parents and babies. Additionally, changing hotels frequently sounds not fun. My husband and I have done flights with our daughter already and it’s been fine but six hours or less.

Do either of these rough ideas sound good? Or are there better ideas? Our goals of the trip are to take advantage of the time off that my sister has while on leave and to enjoy sister time together. Light hiking or walks are good but unfortunately with health problems true hiking is not an option right now. Another goal of mine is to have straightforward travel from SeaTac.