An 8 year old girls cervix isnt very sensitive at all. All you will do is bend the tip of your dick and it will be as strange for you as it is for her. As long as you dont have any foreign objects or sediment in the mix, sex is fine for most girls from 8 and up when the vaginal canal hardens up some to permit growth.
The reason why most people shy away from sex with kids earlier than that is because if your rough, your penis can actually tear into the anal cavity if you aren't lined up just right.
They can bleed out fast and die. I've seen coroner pictures of toddlers and post toddlers where this has happened. I'd never go with a girl under 9 or so again for just this reason. But for god sakes, if you do... use a water not petroleum based lube like astroglide and take it easy. It can be alot of fun if done right