D.W.
That's a tough one. We have always graciously accepted, but we have never asked. Some people like to hold garage sales and bring a little money into their household that they just don't get by providing hand me downs.
You could always say that you'll gladly give them back when done so they can choose what they want to do with them.
If you're not comfortable asking (which I am not), you could always visit Goodwill or a second hand store, garage sales, etc. I have many friends who get great clothes for next to nothing there.
Good luck.