Sorry, when you took out the loan, you agreed to repay the lender. The lender is not going to release you because you used the money for your son and he didn't meet his obligation to you.
Sounds like these are private loans. You should check the promissory notes regarding late fees and collection charges. You should also asked the collection agency for a full accounting to make sure the claimed fees add up and are justified. I suggest you find a consumer attorney in your state who would be interested in double checking everything. If the collection agency has misrepresented the amount or status of the debt, it may owe you money, which you could in turn apply towards the debt. Also, the threat to garnish wages is probably a bit much. If this is a private loan, the only way your wages can be garnished is if you are sued and lose. Not all lenders bring suit. This is another reason to find a consumer attorney in your state. Good luck.