Both of you belong to Ezhava OBC caste and your husband has a wrong caste certificate. The first logical way to proceed is to correct the caste certificate of your husband.
Your husband has two options, one to apply for a wrong caste correction or apply for a new caste certificate.
These are the list of offices that are designated as Common service Centres in Salem by the Government for caste or community certificate services.(https://tnega.tn.gov.in/cscLocations/csc_locations).
This would be the application form (http://cms.tn.gov.in/sites/default/files/forms/obcform_0.pdf).
Your husband needs to have proof that he belongs to the Ezhava OBC by brith, so you need to have your husbands' parents caste certificate, you can enclose his sister's attested caste certificate.
It would be wiser to approach the Taluk office, Salem Town, Salem, 636001. The other option is to approach the respective authorities based on the village he was born. This link has the village name that fall under Salem West or Salem North.(http://www.salem.tn.nic.in/dtaluk.htm).
Please keep the necessary documents in hand (Ration card, DL, Aadhar card, PAN card or Electoral card).
If this is difficult, then meet any good civil lawyer who would be familiar with the process of getting the caste corrections done.
In India caste certificates are issued by the revenue department, the other option here would be to meet the Village administrative officer or Revenue inspector or the Deputy Tahsildar.
Ezhavas are also referred to as Thandans, IIhava, Izhava or Thiyyas. Most Taluk offices will have a list of (http://www.bcmbcmw.tn.gov.in/bclist.htm). Your husband can check this with his elders and or relatives who are in government jobs, who would have used a caste certificate in the past.
Once you get the correct caste certificate, then apply for your daughter (http://www.tnlegalservices.tn.gov.in/forms/COMMUNITY%20CERTIFICATE%20FORM.pdf)