I think it's great that you're taking such an interest in your daughter's horsey-ness and actually buying her a horse when she's ten. I am so so envious of her. What a lucky kid! :)
On the train with the others (just to beat that some more, haha) about an older horse (not saying you should get one this old, of course) but I got my mare three years ago when she was 23 and have been riding her hard ever since. I even find her completely overwhelming, at times, with her love of running and her sassy-ness. Haha
Of course, I don't do anything like jumping her, just trail riding, but we trail ride on asphalt regularly and she's holding up just fine. Most of what we don't do is limited by the fact that we don't have access to a trailer. If we had access to a trailer, we'd be showing like crazy and whupping patooties.
Actually, last year, right after she turned 26, was when I first started her on any sort of joint supplement due to her age and nothing else. I forsee any children I might have, eventually, riding her happily when she's 35 or more (barring any sort of medical issue). As long as a major health issue doesn't stop her, she's looking at being around for at least the next 10 to 15 years.
Just find a well conformed horse, preferably one started late (less wear on joints and such), of a breed known for longevity (Arabs, Arab crosses, that sort of thing), about 10 years old, and you're looking at a horse that, barring a major medical situation that you can't predict, will be with you for the next 25-30 years with ease, even with hard riding.
Good luck! I'm still crazy envious of your daughter. :)