Such and similar queries have been discussed at LCI earlier also.
Your interest is fetch benefits of reservation to the child for Govt. job.
If IT is state Govt.job then carefully check if the respective state govt has issued any order covering benefit of inter-caste marriages and what is covered within the scope of order!
When you apply for the caste certificate the O/o Authority may not agree easily to issue caste certificate on the caste of mother. Based on precedence’s yu ay have to approach court of law.
How and under customs of which caste child is being raised may matter e.g' Mother's caste.