There are four basic signs that will give you indicate of periodontal disease in your child:

Signs of Periodontal Disease

Bleeding
Gums that bleed when brushing, flossing, or at any other time

Inflammation
Swollen and red gums

Recession
Gums that have receded from the teeth, sometimes exposing roots

Bad breath
Bad breath that will not go away when you brush or floss the teeth.