Based on my experience at Katepwa Beach, where an inferior eighteen hole layout with mostly 3's and short 4's but a few longer 4's and 5's was transformed into a nine hole course, I would strongly advise your client not to "downsize" the quality of the golf but rather go with fewer holes, likely nine, of the highest quality feasible. Players who travel to play the course almost always ask, "when will it go 18?", but members who play it regularly (most who have previously been members of eighteen hole clubs) have really come to like, even prefer, the nine hole experience. I know guys who get up and play nine holes and are back to their cottage before anyone else has even stirred, and they can spend the rest of the day with their families on the lake, at the beach, etc. Others play back to back nines from alternate tees. Still others, like myself, will often play nine in the morning with the guys and nine in the evening with their wife and children. My wife enjoys golf but is really turned off by eighteen holes. She and others like her would rather play a "real" nine hole course than a downsized eighteen hole course. They have absolutely no attachment to the concept of eighteen holes. None whatsoever.
Now for what I wish we hadn't done. We had some extra land left over at Katepwa and we created a course with tiny grass greens but otherwise non-irrigated tees and fairways. It's called "The Family Nine" and consists of nine holes, mostly par 3's and a few short par 4's. I get a kick out of watching people play there. Anything goes. Speedos and tank tops. Twelvesomes. Groups that range in age from 5 to 95. But I now believe it was a mistake. We should have gone with a much shorter course that complimented the full length nine holer. It would actually be more accessible to new golfers than the current course and would attract "real" golfers too, while providing a far better opportunity for young players to grow into the game by developing a love for the short game. I would design the holes so that grandmas could tack and bunt the ball to the green, while better players would be encouraged to develop a full repertoir of short game shots. The greens would be really large with lots of undulation.
Most who have posted here talk about "Family Golf" as though it is "Parent and Young Child" or "Golfer Spouse and Non-Golfer Spouse". It's a lot more than that. The ideal facility will accomodate that but also accommodate the full spectrum of family golf and that includes teenage children paying with their parents and their friends, grown children who are home visitng their parents, etc. True, full blown golf is what these families want.
A final comment. If your client is concerned about property values as I'm sure he is, he should definitely go with a full length nine hole golf course. There is an older, shortish nine hole course fifteen minutes from Katepwa. Since the course at Katepwa was built, a very significant discrepancy in real estate values has developed between properties near each course. People want to be able to play real golf and they'll pay to be close to it.