I would buy a second hand boat. The environmental cost of new boats can be huge (rubber or PVC, shipping from far away, etc.) and you'll get 20 years out of a good used boat. For a used boat I'd suggest hypalon. Also, don't buy the discount series boats (Hyside's Rio Bravo, Aire's Tributary, NRS' Otter, etc.) or you'll end up regretting it.
Look for a used Avon, Hyside, or NRS boat in decent shape. 14 feet is the size. It's versitile, family friendly, and has a high resale value. They're also the commonly used commercial rafts so you can find them everywhere.
Don't buy that glued PVC crap (Vanguard, Saturn, Maxon, etc.). It sucks, won't last, doesn't roll well, and given your family's inexperience I'm guessing you don't want to have to repair a seam.
Check out the swap section on this site, as well as on NRS - Kayaking Gear, Rafting Supplies and Boating Equipment
for good deals from fellow rafters. Ask "does it hold air completely for 24 hours?", "are the baffles and I beams in good shape?", and "was it stored inflated?" If the answers are yes, yes, yes you're on the right track.
P.S. If you do buy a new boat I think the boats b-liner is selling might be what you want. Glued hypalon, sold by real boaters, and very inexpensive. He's just over in Salt Lake too, you could pick one up easy....