We did this route back in March. Really nice roads, and good cafes too.
Roads can be avoided for the majority, if desired, Claire.
There is a cycle path between Peebles and Innerliethen, on the old railway. then you are on the wee Elibank road, back on trails from Ashiestiel to Yair, brief bit of B and minor road round the corner and back on to cycle path to finish into Tweedbank.
For a family adventure you can ride the length of the Tweed, from source to mouth 100 miles. Sam and I did it 3 years ago, over 3 days with Kate and girls in support in the van for camping. Its a beautiful route transitioning from Hill and moor, to woodland valleys to golden farmland. 2 days would work too.
Yes, we went out on the road, came back on the cycle path which now goes all the way to Walkerburn. Its a super route!