My kids are adults 21 and 22 years of age. I live in WA state and they live in IN. I'm on SSD/SSI. Can IN still collect back support? If so, then how can they do this?

Yes, if you still owe delinquent child support the government can have your SSD checks garnished to pay for this. The IRS and the Child Support are the only agencies that can touch your SSD checks.