How will blonde highlights look in a 9 year olds hair?

I personally think that is way too young. I'm 15 and I dye my hair at least 3 times a year. But, I do recommend that if you do decide to put in highlights, that they are professionally done. Doing your hair at home can usually be rough, sometimes your hair wont take the color the way that it should, turning it vibrant colors, such as red, copper, pink, green.

Just visit with your local hair salon. Have them check out the condition of your hair (which should be pretty good) and they'll use the correct dye for your hair type. Remember: NO DARK COLORS!

I dyed my hair dark brown, then bleached out the color to add a red tone to the under layers of my hair, and my hair turned pink.

I highly suggest thinking about it for a while longer, make sure it's what you want. The only way to 'fix it' if there is a mistake or something you don't want, is to grow it out.