Sherwood I like your idea someday I might do something similar ... But I have some things to add.
1. I wouldn't try to raise fry to adult in the cage. I think the best approach is to use the cage to seed your pond with fry and just let them grow to whatever size they are able. Your pond can support a very substantial weight of fry and juvenile minnows at various stages of development. Yes they will be eaten by BG and LMB .... but that is point right?. So just try to protect the brooders in the cage so they can make babies. IMHO the mesh should be large enough to allow new hatched fry to go their merry way and small enough to keep brooders in and predators out.
2. Keeping screen clean enough to allow water flow through it is important. Might be pretty tough to do in full sun so is there any way it could be fitted under the dock and fed through the dock in some fashion?
3. You mentioned a kiddie pool and again there is a limit to the number of fry that can be grown to adolescence or adulthood ... BUT ... if you collect eggs and hatch them you can stock free swimming fry into you main pond.
Here's a good reference in case you are interested.
