They cannot garnish money from ssd. They garnish from ssi. There is a diffrence between the two. If a man is dissabled, he is not obliged to pay child support. Now.. They will use ssi to pay child support. SSI is determined by his work history. If he never worked. There is no ssi and no child support. But If he gets SSD, your kids are suppose to get a check also. It's up to him to add your kids on so that they will get benefits too. But if he doesn't, you can report it to Social security. The check they will receive will be separate from his check. That's something that the government pays for. So it's like the governmment is paying child support for him. He'll still receive his full check.
They certainly can garnish wages from SSD for child support, however, as far as the percentage, I would assume they go by certain guidelines according to what you are receiving just as they would in a standard child support garnishment. I would call social security and/or the court and ask them what the guidelines are or where you can get them.