My mother in law is taking my father in law to court she is trying to get back support since my husband was 5 he is now 40 the whole time she lived in California she was on wellfare my husband came to live with me and my family at 15 now she moved to virginia 2 years ago just this month she took my father inlaw to court he is going to have to pay 18,000.00 will that go to her or the state of calif since she was on wellfare that entire time

That doesn't sound right. Only your husband would be able to collect pack child support at this point. The point of child support is for the children, if your husband were still a minor in her care she would still be able to collect but once he became an adult she lost that ability.

I will disagree at no point can the "child" collect, it always has to be the other parent,

But there is a statue of limition on how far back she can go, I would doubt she has a case what so ever. So I doubt that the judge will even hear the case esp he he will get an attorney.
Now he may owe the state back, but they would file their own law suit for their money, and not all of the same statue of limitations would apply to their money.

He needs an attorney.

No, once the child is an adult hey can collect. My aunts just when through this. Their father passed away and they were finally able to collect from the money left to his estate or w/e its called and their in their 50's now. They just had to get an attorney and go through the courts because all of the money was supposed to go to someone else but they had evidence that he'd never paid their child support. And my own attorney told me that if I decide I want to go back and sue either of my parents for all of the back support they owe me I can at anytime.

I am glad it worked out, I have seen many cases where it was not allowed, the idea, the child support agreement is between mother and father, the court order is to pay the mother, ( not the child) the money is just for the use of the child, and if not paid, the mother is using her money for those expenses, so if the money is ever paid, it is still always owed bact to the mother. Also of course most states have a staute of limitation on how long you can go back to collect.

But then laws change all the time, and laws vary by US state