A. In India, when it comes to the legal status of the caste of a child born out of an inter-caste marriage, traditionally the child takes up the caste of the father.
The are a few court rulings in India, which state that the circumstances under which the child is brought up also influences the caste to which the child belongs.
For instance, if the father belongs to a foward caste and the mother belongs to a SC or a reserved group, then the mother can prove that she has brought up the child as per the norms of her community for the past so many years and then claim the reservation caste status (the mother's caste) for the child.
B.Can child be reserved caste if mother belongs to reserved caste while father belongs to scheduled caste if both willing to agree?
Yes, you need to ensure that you have enough evidence that the child is brought up by the mother as per her reserved caste customs and practices.
C. If yes what is the procedure to get caste certificate?
Meet the registrar of births in the municipal office/thasildhar that has jurisdiction over your home locality. You would need the birth certificate of the child.
If you don't have it, this is the form for birth certificate application in Telengana
For the caste certificate (http://tg.meeseva.gov.in/DeptPortal/Application%20Forms/REVENUE/Community%20and%20Date%20of%20Birth%20Certificate%20relating%20to%20Scheduled%20Castes%20and%20Back%20ward%20Classes(II-A).pdf)
D. Lastly, If you face any opposition or the request is rejected, then you need to seek the services of a good lawyer as some officials may stick to the old tradition which is not a mandatory norm ( child taking the caste of the father). If approaching the court, please state your reasons clearly and the past rulings related to this, you should get a favorable response.