Tonight with your 5 to 8 year old
The prompt
“What do you think should happen before someone walks into your room, knock or no knock?”
What to listen for
If they clearly want a knock-first rule, they're expressing a real, healthy desire for their space to be respected, which deserves to be honored, not brushed off as silly.
If they don't mind either way, that tells you privacy isn't a big need for them right now, which is just as valid an answer.
If they say "I don't know," ask how it feels when someone walks in without warning, to get at the feeling behind the rule.
What this tells you
How strongly they want a knocking rule shows how much they're starting to need their own private space to be respected.