My children are both under the age of 10 and like swaps. We always carry a pocketful of Toy soldiers and similar items. I recently bought a bag of 50 soldiers for £1 at Trago Mills. My children particularly like the more generous and unusual swaps. Things like Foreign or Old coins and stamps go down really well with my two. I think swaps in the family caches in places like cadover, dartmeet, burrator etc is great. But there is not much point at cut hill or ryders hill and places like that as young children wouldn’t really visit too often.
Even if I’ve been caching on my own I’ve often thought of the children if I’ve found something interesting and made a swap.
I do know what you mean about rusty, smelly toys. Recently I travelled some distance away from home and got a cache. I had a TB that I needed to drop. The cache was more than adequate size to fit the TB but was full of toys of varying quality. (Not sure if it was right) I removed a sticky bug thing with one eye. A badge that was rusty and a key ring minus the ring. I was then able to fit the TB.