Hi Andy
Are you talking 10 mt precision or FT?

If its 10mt then the choice is very limited at 12, as they are generaly just that little bit too small. I coach a school team at 10 mt and we find that the best bet are classic match rifles shortened and lightened. We use Original 66's shortened and the muzzle weight removed for our main squad but we do also have some AA200 target versions for the smaller development ones. They are very light and although they need the trigger carefully setting to get the best from them they are adequate.
I have also made a Junior FWB 300 by shortening the stock and removing all barrel sleeves and shortening the barrel, this works beautifully but is still a tad too big for 12 year olds, the good news is they grow very quickly, so don't spend a lot as you'll need to change it very quickly.
Good shooting
Robin