Suits involving the support of children are heard in family court. The bill was for the child, not the grandparent so the grandparent is not the proper defendant.
Have an attorney review all the facts of the situation and file appropriate motions to dismiss the proceeding
If you are not paying child support , then it may be worth it to lay based upon how much you have saved over the years. If you are sued for child support, then you may be liable until the child turns 21. Mother can also be sued by custodian of child for child support. Up to custodian to commence a child support case.
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From the information you provided, the mother is equally obligated for support, and the grandparents would presumably have to file against both of you in family court for support related expenses, as this is one. However, if the mother and you are paying no support, and this is all the GPs want, you are getting off cheap.
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