Welcome aboard, Michelle! What an amazing summer you'll have as your family gets to enjoy a magical cruise with Disney in Europe! I was so thrilled with all of the experiences I had in the Mediterranean with Disney Cruise Line last summer. Truly, it was the trip of a lifetime, and I just know you'll have amazing experiences as well!
Car seat laws vary by country, so if you plan on hiring private transportation, you'll want to research the laws for each country as well as chat with the company with whom you'll be receiving transportation. Regardless of the laws, I completely understand that even if the law says no car seat, you will want your little one to be safe! I feel the same way! For this reason, I suggest you stick with
Disney Cruise Line's Port Adventures. They always partner with great excursion groups that will provide the best experience possible. Most of the time, guests will be riding in a motorcoach, though you'll want to note each specific Port Adventure will list the modes of transportation. These motorcoaches do not have a way to latch in a car seat, so most guests will hold their little ones on their laps. Even in the Bahamian and Caribbean cruises, this is the norm when traveling to the adventure on a motorcoach. I've always felt safe on these high riding motorcoaches, even without my kids in a booster or car seat.
If you have questions about specific adventures or details, I'd encourage you to contact Disney Cruise Line's agents at 800-951-3532 who can give you specifics. Just remember that Disney works hard to ensure the very best Port Adventures for safety as well as entertainment and education for their guests, so you can rest assured they're planning for families.
I hope those details help as you continue planning, Michelle!