How is it that SSI can reduce my daughters payment, she is permanently disabled for life, using the child support paid by my 14 year old daughters dad who is not the disabled child's father? Why is that counted against the SSI?

SSI amounts are determined by income of the household, not for each individual.
Almost all social programs, even the non-welfare ones, are tied to each other, and to things like alimony and child support, part time jobs, every penny coming in.
The taxpayer is hurting and objecting. All you can do is make sure you are using all possible programs for housing, food, and health.