All other factors being equal, the best location for your air/heat vents depends on the climate you live in. If you are in a northern area that gets cold winters and your greatest expense is heating, floor vents will save you money as warm air rises from the floor to fill the room. If you are in a southern location where air conditioning is your greater expense then ceiling vents will save money as cold air will drop to fill the room.
If either situation is reversed your blower will have to work overtime to keep the air temperature comfortable...example if your cooling with floor vents it will be very cool at floor level and your A/C will have to work harder to get the cool air up where you want it.
Manufactured housing is proving to have many benefits that come from assembly standardization. Most factories have to pick a single standard for duct work... either overhead or in the floor.
I am in Florida and Silvercrest is exclusively build and distributed in the west so my experience is limited to what I have read about them, and I must say it has all been very good. They do have 2 factories and your dealer probably only deals with one of them. Here is the factory contact info
Corona, California 1-800-382-0709
Woodland, California 1-800-264-0010
Give them a call and find out for yourself what is possible. Ask if they offer factory tours and see for yourself how their homes are constructed
I understand the problems of children and floor ducts. I had both at the one time and I'm here to tell you that crayons dont do well in the duct work with the heat going. But again, that is only one factor in your energy efficiency. Compare all of the building features and particularly the insulation package and upgrade insulation where ever possible. This will be an investment that will pay you back many times.
Palm Harbor also makes an outstanding product. Skyline is one of the oldest builders around. The have plants across the country and build everything from the most basic to the high end homes. When you consider Skyline, or any other builder, be sure that you include plywood or OSB floors.
For a highere education consider The Grissim Ratings Guide to Manufactured Homes - This book rates every Manufactured Housing Builder's products, price ranges, construction ratings & more. See more details HERE
Here are a few photos of 4 section manufactured homes.