I am on ssi I get just enough to pay rent and child suport saya I owe them can they take it out of my social security ssi monthly check

Yes, they can. If you cannot afford the support, go to court and ask for it to be reduced.

If you are on SS Disability you can go to the Social Security office and sign your child up for them to receive their own disability check. If it is SSI or supplemental income then you cannot "sign" them up to get their own check.

Is it ssi or ssdi? If it is ssi then no they can't. You need to go back to the same court that ordered the support and show them the proof that you are receiving ssi and that is the only income you have and they should discontinue current support. However, any past due support that is in arrears you will still be held responsible for but they can't take it out of your ssi.