I realize this is an old post and hopefully you have all your travel arrangements made by now. We have been traveling to Europe at least once a year with our three kids since the oldest was an infant. On our last trip to Germany I bought the two older kids (3 and 5 at the time) a blank spiral bound water-colour/sketch notebook to use as a an art journal to document their trip.
I bought a plastic 4X6 flip top photo box for each of them and filled it with crayons, small markers, a glue stick, kid scissors (those were packed in checked luggage on the plane) and some stickers - they could pull these out on the plane or in a restaurant to work on their journal or just colour without things rolling all over the place. I also brought along a pencil case for myself with special markers, a small set of water colour paints, washi tape, special stickers and a few other things for special touches and unique pages. I also bought a Fujifilm Instax Polaroid Camera with enough film for 2 pictures of each kid per day (the washi tape was the only thing that worked to keep the pictures in the journal).
We worked on our journals each day ... drawing pictures of things that happened that day, cutting up brochures and maps, adding stickers we found at souvenir shops, coasters from restaurants, boarding passes, postcards of memorable places, etc. Clearly I did a lot of work on the 3-year old's myself but it was still based on her experiences and ideas. Each day I would ask them what the best part of the day was and write it in the journal. During and again at the end of the trip I asked them other questions about their favourite experiences and memorable things. All these things I noted in the journal for them.
When we got home they were so excited to take their journals to show their Granparents and other relatives. We have been home for several months now and they often ask to look at their journals. I keep them in a safe place but bring them out to flip through whenever they ask. The journals have really helped solidify the memories of our trip for the kids and have become very special keepsakes. We are planning our next trip now - Italy this time - and I am already thinking about what I might add to their journal kits.
Have an amazing trip. You will never regret traveling with your kids.