OK, at 850, the kids SHOULD be eligible for a child's benefit.
It can be as much as 50% of his amount, subject to family max which would be 150%, most likely - meaning, if that applies, your kids would split 50% of $850, potentially.
You simply go to the SSA with photo ID, their Birth Certificates that show he is their father and you are the mother, and tell them you want to file for your children to get their child's benefits.
If the judge didn't order CS be paid from his 850, it is probably because that is barely enough for him to stay alive on, much less support others. Any divorce / custody lawyer could help you review your state child support laws. You can also google them, if you are an adept online researcher. But, again, if his total income is 850 and he doesn't have a lot of money otherwise in the bank or lots of assets, I would not expect him to have to necessarily pay support - your better avenue would be welfare if you also are not able to fully support based on your own income. But, go get them any SS they are eligible for. If they are eligible, you can get up to 6 months retro as well if he has been receiving SSDIB at least that far back.