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).
This route crosses over with the V90 at Montpon and Perigueux, and can also link to Thiviers from Perigueux.
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!
Full route overview here
Download GPX file here