The Picture People does really great children's portraits. They offer simple backgrounds and focus on your child's personality. The best part is that your pictures are ready in an hour. The hardest part is that they print out all of the pictures, so you know that the ones you don't purchase will end up in the trash. Torture!
At $18 a sheet, the prices are a bit higher than some places, but they offer a great portrait club and lots of coupons. Make sure you sign up on their website to receive coupons.