So we learned a few things doing this last trip.
Our kids, at this age, do not do scenery. I doubt many kids care about scenery. We spent a bit too much time driving around beautiful places and would plan for less of it next time. Driving with bored kids is never fun . Audio books are the best way we know to pass the time in the car.
It's better to book accommodation in advance. While booking at the last minute offers a lot of flexibility, it also wastes a lot of time and finding room for a family of four is harder than if you're just a couple. Next time, we book it all.
We learned that we can't take our kids somewhere, show them something historic and then try to get them interested in it. It doesn't work that well. They need to know something about what they're going to see before we get there and then they're more likely to be interested. This means building in more time into the itinerary for learning about what we're going to see. See less and understand more.
If you travel Europe and you need Internet access, take a laptop. Internet cafes are few and far between but wifi is everywhere.
With kids it's all about "doing" and activities. We have to keep reminding ourselves that and not just rushing around and "seeing". The kids always seem to like climibing up to the tops of places.
On a long trip, we need to plan on more free days where you don't have any plans and can just take it easy and get the laundry done. We tended to have these days when someone ran out of underwear!