If she is active and healthy...then even though she looks lean..it's fine. Slowly add things to her food plan. Are you feeding milk outside? Present food in an interesting way. Tell her that...her favourite cartoon. character have said to have food. Her favourite cartoon character is strong coz...he is having dal rice. If you are giving chapathi or dal rice every time...then change that. Give her Tiffins... like idli, vada, dosa ( many kinds of dosa are there) , sabudana, khichdi, pongal, palav. Give wat she likes. if she have the habit of eating chocolate, first stop that. Give her fruits. dry fruits.