While I didn't travel with any young children I was on the holiday Danube and there were some younger children who appeared to be in the 6-10 range. For the most part they were very well behaved and travelled and seemed to enjoy themselves. My only caveat would be to possibly not try to make them do every single activity. I saw a couple of kids falling asleep at dinner.
I'm guessing there might have been even more on the summer trips-there was a USA Today article about the cruise and there seemed to be lots in the photos. We enjoyed it so much we're doing the Rhine this Christmas.