How can I tell the difference between scratches from playing or scabies?

Location of sores. Scabies causes intensely itchy small bumps that tend to occur in skin folds (under the arms, below the waistband, between the fingers) if these area are not involved, your child probably doesn't have scabies. Scabies lesions will also occur in a line or track at times.
Scratching in sleep? Scratches from playing usually do not itch and lead to additional scratching. Scratching from scabies is usually worse at night, persistent for days to weeks, and associated with a red, aggravated rash worse on the arms, legs, hands, and feet. Also, scabies rashes sometimes have "blisters" or fluid filled bumps. It is one of the rare causes of rash on the palms of hands or soles of feet.
ITCHING. The biggest difference is probably the intense itching associated with scabies. Scabies lesions also do not "heal" the way you would expect a scratch from playing to heal. Scabies lesions can occur anywhere on the body, but often are found between the fingers and toes, along the waist band, under the arms, in the neck or other areas where skin rubs against skin.