We kept our bikes that we bought in the UK and shipped them home afterwards (to Australia) and they make great reminders of our wonderful days in Europe. We are glad we kept them.
We found the infrastructure along the Mosel superb and quality campsites aplenty. The kids had no problems completing 35 miles a day (albeit with some help).
We went for overkill and bought adult bikes that would take all the weight of a family touring and camping for 6 months with school work to boot. Whilst it is easier to ride a bike than get them on or off a train we never had any problems with this. On German trains you book a 'seat' for your train, so just plan ahead a little.
www.4bikes1tent.org
Damon