I am a parent and I can confirm. Most of my 4 year old son's toys were purchased for him by grandparents, friends, etc. We've bought almost none of them. We have bought him a lot of books and good second-hand clothes.
Well, parents still have final say. My parents won't give my kid anything I don't approve.
So, if their best friend wants to give them a toy, you just tell them no? I'm sure that won't make them resent you at all...
That works fine for adults, but it's sketchier with kids. Maybe a better system would be to make them trade toys and choose which ones they keep?
No, I'll tell their best friends' parents to tell their kid to only trade toys with my kids but not give them out. I recall my parents and a good childhood friend's parents having a similar conversation when I was 10 or so
Yes, we've told family/friends that instead of buying something that will be garbage in a few weeks they should spend the money on an experience with the kids (books are an exception). Most people have understood (except grandparents, they seem the most resistant to the idea).