A child's income is never filed with the parent.
so a separate return
I'm not sure why you elected to have a child's SS benefit have voluntary income tax taken out; unless the child has substantial other income, the Social Security benefit for a child is almost always tax free. The child would need to have an income besides SSA more than $10K.
The child will have not taxable income, so a refund of all tax paid in would be expected. In addition, you should contact SSA to have the voluntary withholding stopped.