How is the amount of SSI awarded determined?

I have a daughter and son with Autism. Both applied for SSI having just turned 18 years old. They were approved, but the amount they were awarded monthly was considerably less than I thought they would receive, as there is no way anyone would be able to live on the amount given to each of them on a monthly basis. They live with my husband and me right now, but we also have two other children, and my husband is the only person who works in our household. They are both low functioning, my son (at age 18) is still in diapers. I don't understand how they could have decided on such a low monthly amount - isn't is supposed to cover shelter, food, medication, clothing, doctor's appointments, basic costs of living? Why would my husband's salary have anything to do with this, we aren't going to live forever, then what happens to our two beautiful children/adults? It is so scary. Please answer at your earliest convenience, thank you!