DH & I have keys to our parents’ houses - he has a key to both my parents’ house & his mom’s house, & I have the same.
However, my MIL would probably prefer I didn’t have a key & doesn’t remember she gave me one. And, she now locks her storm doors which are a different key than the other doors, & she hasn’t given us a key to the storm door so it doesn’t matter anyway. However, she says she’s given us a storm door key - She wasn’t home on Christmas Day when we brought her present, & the storm doors were locked. DH called to see when she’d be home & she’d told him that he had a key & could go on in. He told her we didn’t have a key to the storm door, & she replied, “Oh, I thought I gave you one.” No, she didn’t.
Even w/ keys though (MIL’s storm door notwithstanding), we don’t go over to either house & just make ourselves at home if they’re not there.
If we know they’re at home, we ring the doorbell first.
If they’re not at home, we never go over to just hang out or whatever; we’re usually dropping something by or bringing in the mail when they’re out of town. When my parents go out of town, we usually leave some sort of fun surprise for them. If the kids are w/ us, they may grab a snack at my parents’ house. When my father-in-law was alive, we would go over every now & then on Sunday evenings, &, sometimes, they wouldn’t yet be home from church. We’d go ahead & let ourselves in & wait on them.
My parents also have a key to our house & will feed our cats while we’re out of town & also always leave some sort of fun surprise for us. We’ve given my MIL a key, but she doesn’t know where it is & would never come over to our house w/o us being here anyway.
Our kids will always have keys to our house.