Do you get SSDI back pay for your child's auxiliary benefits.?

Yes, your child can get back pay for auxiliary benefits depending on when you applied and when you were found disabled. The same 5 full month waiting period from the disabled onset date applies for auxiliary benefits.

As long as you listed your son's name on your application for Social Security benefits, he will be entitled to back pay for the same months that you are. Although Social Security will not process a dependent's claim until your claim has been decided, any child that was listed on the initial application gets credit for the filing date of your application. If, for some reason, you did not list your son on your application, he can only receive a maximum of 12 months past-due benefits from the date the dependent's claim is filed. But that's rare, because Social Security's standard procedure is to ask you for the name(s) of any children under age 18.

I think you previously asked about Rep Payee issues so I agree with the prior responses but just want to add that it is best to think of the child's auxiliary benefits as just that...the child's. There are many instances when people seem not to understand that the Rep Payee for the child's money will merely be managing the funds as the money will always belong to the child and only to the child. SSA will typically appoint the custodial parent as the Rep Payee for a child's check. If you are under any sort of child support obligation, then consult with an experienced family law attorney on whether there needs to be a modification of the support obligation and remember that IF this is the situation, then the court won't change the order unless specifically requested and can never make retroactive adjustments so failing to modify when it is appropriate can potentially cost the SSDI recipient a lot of money.

You should be eligible for auxiliary benefits for your son, so long as he is your dependent. (Basically, he lives with you and is a minor). I've noticed that Social Security sometimes takes longer to process auxiliary benefits and I would recommend calling them to follow up directly.