Holy Black .22LR

[Be-A-triss Bandit]

There is a Rossi or Handyrifle 410/45 Colt single shot -- may I say 45 Special?  Sounds like that's what the cartridge should have been designed for; starting a kid in BP loads.  Have the barrel lined to 25-20 or 32 if you don't like 45.
The 22 route is just to much trouble.  Did it as a kid, shot maybe 3 or 4.  Don't recall anything else so it couldn't have impressed me much.
And I still have my Ithaca 49 Saddle Gun - My first gun.  Slender barrel version with the dovetailed sight and dovetailed fake magazine tube support.  Won't give it up either.  I'd have a nice walnut blank set made if it didn't detract from the collector value.  Gee, did I actually say that?

[Dakota Widowmaker]

A 32 H&R as others have suggested would work quite well.

The other option would be a 38 long colt with 125gr bullets.