Hi H.,
Tea Tree oil might be a bit strong for a baby or young child. They are very concentrated chemical constituents of the plant. The amount of drops you use depends on the age and weight of the child. Generally 1 or 2 drops of one oil is best. Organic oils are the best if you can get them. Very gentle oils such as Roman or German Chamomile, Lavender,or Sandalwood. Books by Valerie Worwood, Shirley Price and Maggie Tisserand all have information about how to use EOs with babies and children. A website to buy oils is Aromatherapist USA. Have you considered that the dry hair might be a result of what you are feeding your child? The condition of our hair reflects nutritional status of our bodies. Hope this helps.