Live in an estate, good few children. I've always said to my children that if they want to knock for their friends they should ask them to either come out to play on the path/garden, or to go down as far as the green once they are older, or ask if they want to come into our house. So my rule is you can't knock and ask to go into your friends house. You have to be invited.
My DD (7) is friends with 2 little girls across the road. Another little girl sometimes plays with them but my DD doesn't really know her. This third girl has just knocked on my door with one of the other girls and asked "Can we come into your house to play?" She is only 5 I'd say, one of the dads was watching from across the road so I let them in. My DD wasn't too enthusiastic but it felt rude to turn them away when my DD might be out playing with her actual friend later.
AIBU that if you send your child knocking on doors you should tell them to invite people out, not invite themselves in?