Hi, can the ssdi child benefit be used to offset the child support arrears that had accumilated before the onset date of disability?

By offset, do you mean used to pay the arrears? Who has the child? Who legally has the child? What state is everyone in? Generally, under federal guidelines to the states, the answer will be yes, they can take SSDI child benefits. If it's a windfall benefit(a large retroactive amount of SSD) they can take all of it. If it's the monthly amount ongoing, you may be able to discuss the portion taken with the court.

Oops sorry, guess I need to be more clear, my ex has not paid child support since 2005, he was deemed disabled in 2009, my son got a child benefit for the time of 2009 to present! 2011, and is now collecting monthly checks, my ex still owes arrears from 2005 to 2009 and feels I should give up the arrears and not collect it, we went to court with the child support agency and they stated that the child benefit can not be used to offset the arrears before the onset date which is April of 2009, so now he has appealed this and we have to go in front of a judge, so I was wondering what are the odds of the judge ruling in favor of my ex and me losing out on my back due child support! My child support attoney said that Ohio does not aloud the child benefit to be used for arrears before onset date, can this law be over thrown?

Your attorney will argue your case within the framework of the law. You don't 'overthrow' laws. Even following the letter of the law, the court will weigh the current situation. If the arrears stand as still owed, they may allow a payment that reflects his ability to pay. If he gets off SSDI or inherits or wins the lottery, go back to court again.