Patience is your best tool. Wear a bonnet, a hat or something on your hair to hide it while you're waiting for it to grow back.
My sister also did that to me when we were kids. She cut my bangs real short; I looked so stupid! I cried to my mom to have it repaired. I ended with a boy's bob cut.
Since then, I steered clear of my sister when she's got a scissor in her hands. ^___^