KST 8 Fan--Had to go through the same process myself, with my #1 son, who was smallish and slender. We found that a youth model rifle was just the ticket. Several makers have them now (Only Win when we did this.)
We decided on .243 Win as the cartridge: Accurate, easily available, easy to reload, light recoil, 100, or 95, grain bullet works on deer. Every deer my son pointed his .243 at was DRT.
As to Discern's concern with bullet overtravel: Isn't one of the rules, "Be Sure Of Your Target--And Beyond""?? And in KS you do have some wide-open spaces for a longish shot.
Of course #1 son grew out of the "Youth Model" stock, but since the rifle is a cut-down Model 70, an adult-size stock was easy to find. It's still his favorite deer rifle.
Enjoy the search! Be sure to include the daughter in the shopping and the choosing, and practicing, as well as the hunting. All in all, makes a fine dad-and-daughter project IMHO!
