You need to contact an attorney in your area. It would appear that mother did not disclose the entire arrangement the two of you have as it relates to your son when she applied for benefits. My guess is she claimed full custody and probably indicated that she received no direct support from you, which is true. I am further guessing that she did not disclose how much time you have withyour son or your financial cointribution to his upbringing. If she goes to court with you and states the truth, it could very well have an impact on the benefits she receives.
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The financial assistance that she is getting may have prompted the county to sue if they determined you are paying less than the child guidelines amount of support in the absence of an existing agreement or court order. The action will not take money away from your child, but will possibly take more money from you. It helps that mom is an ally, not an adversary, but that will not be determinative. Try to get an attorney and see what s/he can do.
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It could be that the support they, social services, seek is for medicaid. If you or your ex do not provide medical insurance the county would be looking of reimbursement from both of you. You should speak with your ex about what she may or may not have said to the county and then seek the advice of an attorney.
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