For beginners, plastic pieces are fine. No need to pay attention to the weight, which is just some inserted lead. Do NOT buy colorful ones. They look childish and will be painful if you stare at shining red or blue pieces for 5 hours (that's normal in large tournaments).
The board color is more flexible, but still prefer dark ones: green and black are two popular ones.
If you have questions, go to some tournaments and observe. Check out which one is more common and which one is more appeal to your child.