The kids who live below me (duplex) have figured out how to unlock my front door

I (39f) live with my coworker/roomate (36m) on the second floor of an old house that was converted into a duplex.

I’m going to make up names for convenience and to protect our privacy. The first floor is a single mom (Maria) with her 6 year old daughter (Leah) and 9 year old autistic son (David). I have a good relationship with them. Both of her kids are at the age where they really want to play all the time.

Unfortunately they don’t have the greatest boundaries and I am really terrible at setting boundaries with anyone including kids.

A few days ago when I got home from work I told Leah I had to eat dinner but could play outside with her for a little bit once I was done. I locked the door (because David will just come upstairs if it’s unlocked) and went upstairs to eat. I heard a noise from the stairs and when I looked my front door was open. I went down confused because I thought I’d locked it and Leah said it had just opened when she tried the knob. I locked the door and went back upstairs and a few minutes later heard the door open again. I went back downstairs and asked Leah how she’d opened the locked door and she showed me that If she stands up and pushes the knob with her body weight it somehow flips the lock and unlocks the door. I told her she absolutely could not do that again because it was really dangerous for me and immediately asked my roommate to tell the landlord that we needed to fix the door.

It’s almost a week and the landlord hasn’t done anything, they are pretty flaky like that but they haven’t raised our rent in years so we live with it.

Unfortunately today after I got home from work and locked the door, I forgot to throw the deadbolt. After being home for about an hour I thought I heard something from the door. I went downstairs and found my front door closed but unlocked. It was pretty late (like 10pm) so I didn’t want to knock on Maria’s door and I was honestly too upset/angry about the violation of my privacy to trust I wouldn’t say something I’d regret.

What should I do? I don’t have kids of my own and don’t really know how to talk to Leah and David so that they understand they absolutely cannot open my front door. English is not Maria’s first language so the language barrier might be a challenge trying to navigate this. I’m worried I might forget to lock the deadbolt and the kids will flip my lock and my place will be unsecured while I’m at work. We live in a bad neighborhood and there have been thieves/break ins over the years.

Any advice would be much appreciated.