When Lawrence of Arabia was 19, he went cycling in France following the path Richard the Lionheart (Richard I of England). Here is the route he followed.
Distance: 300-400km (there are a few variations on the official route with some 'sportive' options, but I've kept things simple here and mapped the core 329km from Castillon to Chalus).
Difficulty: This is a long and varied route. It is not flat - there are hills, though they should be achievable by most cyclists if you take your time. Kids may need to push!