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Back benefits for children are paid in three installments and over a year period. However, by now, you should have received at least the first installment. Accordingly, do not call but go to any SSA office as you will get the information you need a lot faster. Note, part of the holdup could be if you earn too much per month. Or the SSA has not yet received a copy of your tax return from the IRS, Regardless, go to any SSA office as that is the place to start.