My daughter bites her teeth profusely while sleeping, what should be done to prevent the same, she is 8 year old.

That depends. Is she damaging any permanent teeth? Some children will start grinding when they are teething. She is the right age to start loosing her baby canines and molars. Consult her pediatric dentist. If no damage to permanent teeth then just monitor. If damage to permanent teeth then night guard is indicated.
Cause?? Dealing with a grinding issue in a child is difficult. If a cause can be determined, that would help- allergies, tooth problems, etc. Making a mouthpiece for grinding for a child is rare since they are in their mixed dentition stage where baby teeth are being lost and new one's erupting. You do not want the mouthpiece to impede the eruption of the permanent teeth.
Nothing maybe. It is common for children to grind their teeth and it can be quite loud. Unlike adults, children are growing rapidly with loose teeth. Grinding helps get rid of the loose teeth and adjust the bite as the jaw grows. Rarely is the grinding so severe treatment is necessary. Mouthguards don't work well on children because of their rapidly changing bite. Opening the bite with resin restorations help...
Tonsils . Grinding is common in children. And can be related to growth and development or stress. But if your daughter snores, or if she has been diagnosed with adhd then there a re two things you should do. First have her tonsils and adnoids evaluated, large tonsils can be a problem. And have a orthodontist evaluate her. Sleep apnea would be a concern .
Tooth grinding. This is called bruxism and has many causes. You should consult your doctor or your dentist may be able to make a splint to guard her teeth.