You will hear a lot of tlak about the 243 and deer running off. I can say I had a few head for unforseen pastures in my youth. However, we also found that it was the bullet as others had mentioned that was the culprit. I started loading my own rounds for it with help from my pop when I was around 10, and shortly afterwords only had one or two run more than about 20 - 30 yds before piling up.
Originally I used the 85gr Nosler Solid base, but when they came out with the 95gr BT' that is the one I went to. I have taken several deer, and loaded for several other folks who's kids used the loads to take some really nice deer as well. None had any problem with magic disappearing acts. The key with this or really any caliber, is shot placement. There is no substitute for practice, and if the youth doesn't feel comfortabel about the shot or shooting then don't force them. Make it simple and easy for them as well as respecting that they can't handle 20 rounds in one secessin like grown ups. Just a few then plink with the 22 or vice versa. The practice putting the shot where it needs to be is the key.
As for the kid's buck, never had it scored. We're just not into that sort of thing. If they look good and mature then thats all that matters to us. She has let deer walk that I know grown men would have shot in heart beat. Yes we definately got it mounted and it makes a great addition to the other one she got a couple of years before, two weeks before delivering my grandson.
The two of them are the best deer we have taken from our family place in nearly 30 years, and she worked hard for them both. I am hoping that the grandson will get his chance this year with a litle help from Pawpaw, and his little 243.