Although it may be true that Jem and Scout never gave Boo anything tangible, they did give him a reason for him to stay a part of their lives--no matter how distant. Whether the children knew it or not, Boo had become their protector, and his concern for them must have given him great pleasure. The children were his only contact with the outside world, and it gave him a reason to show his human side outside the Radley house. They gave him a reason to exist again and, once the threat of Bob Ewell was over, he retreated back into his secluded world again.
First of all, for what it's worth, this is in Chapter 31.
Literally speaking, Scout is right. Boo put the stuff in the tree and they did not give him anything in return.
However, I think that they really did give Boo something. I think that they gave him a connection to the world outside his house. I think that, by watching them and (sort of) interacting with them, he came to care about the world and participate in it. I think that you can see that from the fact that he cared enough to come and save them from Mr. Ewell.
So maybe they didn't give him any material stuff, but I think they gave him a huge psychological gift.