... Why four Par 3's? Well I've been speaking with a couple members and they aren't sure a 9 hole course can hold a 6 year old or younger's concentration for that long. I believe that short Par 3's will not only help with this, BUT it will also probably help the pace of play. ...
You're right, attention span may be a problem for pre-schoolers and primary grade kids.
Last year I read about clubs that have a 5-hole or 6-hole beginner's course. Five holes is plenty for a lot of young kids. If they get unusually motivated, they can play it twice.
On hazards: a good water hazard would be a creek which the kids have to cross. Maybe 50 yards in front of the green on a short par 4. The idea would be to get the kids used to hitting a ball across water without looking up.
For sand traps, a couple of greenside traps would be good, with a small lip. For most greens, dips and rolls around the edges to teach them about chipping, or putting from just off the green, should be popular.
Also, you could have multiple tees for different age groups, including "big kids" tees for adult beginners.