Planning a trip in Scotland with family

Hi, I am looking to plan a one week round trip in Scotland this summer with family (5 people), looking for recommendations for routes. The youngest is 12 years old, so we don't want to make it too tough of a trip, we're thinking around 40 km per day.

Another issue is that we're coming from the Netherlands and aren't sure what's a good way to get there with our bikes. We're looking at taking the ferry from Rotterdam to Newcastle-upon-Tyne, and then the train to Glasgow, then starting our trip up North from there. What do you think?