There is a mini gensis bow that goes from 6-12 pounds draw and only has a mass weight of 2 pounds and is made for small children. If he/she can not get 6 pounds it can probably be worked into fairly quickly.
For a five year old I would *seriously* suggest you try and find one they can try and pull before purchasing a bow. For every single one of the 7-8 year olds (and most 9 year olds) in my junior program 12 pounds on a compound is WAY to much for them to draw without serious training (most are drawing in the 5-8 pound region on a 62 inch recurve and usually short drawing for the poundage - about where a compound would break over). Most of the bows suggested above have the low end poundage in the 15-20 pounds range, and for that age a 15" inch draw may very well be dangerously behind thier ear (the banshee is 19"). They are typically made for an older shooter than 5.
As someone else said - for that age don't fret it too much. Shooting with dad is probably the best part. The rubber band bow is most likely as good as anything else until they get a little older.
I normally do not try and give parenting advice - no kids of my own (the best I have is my junior program). Nor does it normally do any good, if you know this already it's useless to add and if you don't, most do not listen. I've seen this happen too often and I usually add remember - make it fun. At that age much more than "anchor" is really difficult, even anchor usually is. Make it more bonding with Dad than anything else and you will have a lifelong shooting partner.