FYI, a data point for other folks who may find themselves in a similar situation. The Cardiff Marriott in Wales is one of the properties I was considering. The MR website indicates the cutoff for children is 16 at this property (up to 2 per family room with 2 adults). So, I emailed the front desk and inquired if it was okay for a family of 3 (2 Adults+ 1 x 17 year old) to stay in a single family room. They responded back very promptly that is was fine and that they had put a note in my reservation to honour it with 2 Adults and 1 x 17 year old.

So far this is the only property in our itinerary that I needed to inquire about acceptable family room occupancy. Bottom line, it never hurts to ask politely. While not a guaranty of success, your request may be granted. YMMV....

--Jon