My husband and I just returned from a Uniworld River Cruise last Sunday. We sailed the Danube from Budapest, Hungary to Passau, Germany on the River Baroness. We, too, knew very little about Uniworld prior to our trip, but would sail with them again in a heartbeat. The trip met our needs far more than ocean cruising, but the make-up of your multi-family travel group IS important to consider.
The youngest on our cruise was a 9 year old male and the oldest travelers were in their 80's. Due to the strong emphasis on historic excursions, the teens in your group would fare best if they had an interest in history and didn't mind waking at early hours (most excursions left the ship at 8:30 or 9). As one of the other response posts indicated, the typical glitz of ocean cruise ships is not part of European river cruising.
As for the more elderly in your group, walking is a definite component of the excursions. Much of this is done over cobblestone streets which can make it more challenging for some. All of our excursions were conducted by incredibly knowlegeable local guides. Our cruise director was observant and once she sized up the physical abilities of our passengers, she made sure that one of the local tour guide groups (we were usually broken into four groups for walking tours) was designated as the "slow group" for people who needed or wanted to walk at a more leisurely pace. This turned out to be a GREAT idea and helped to accomodate everyone's needs. Any elderly guests you may be including who need to use mobility devices (walkers, canes, wheelchairs, etc) would find the excursions VERY challenging. The terrain of visits to the "old cities" at each stop make this so.
As for overall quality of the cruise... our cabins were small, but adequate. The ship was immaculate, the food superb and the crew extremely friendly and accomodating. Although our ship held 120 passengers, there were only 83 onboard. The atmosphere was casual and intimate. We met many fascinating individuals due to the open seating at meals and by the end of the cruise many felt like extended family. My husband and I both thought that Uniworld's prices were incredibly reasonable and that we got far more than we expected. How often can you say THAT??
You may want to do a search of Uniworld Cruise Reviews. I have done that since returning and have found that other travelers' reviews mirrored our experiences. Some of the reviews go into great detail that you may find helpful.
All in all, I would recommend Uniworld without hesitation. What you REALLY need to consider are the needs of the individuals in your group and decide whether or not this type of cruise and what it offers is for them. Good luck with your decision!