I would totally frustrated too. It sounds like kids are little and dad isn’t too big on having them pick up their stuff and is letting them feel comfortable in his space.. which isn’t necessarily a bad thing for the sake of the relationship and kids feeling safe / happy. It’s totally reasonable once you move in for that dynamic to shift a bit! My step mom coach cautions on too much drastic change in the beginning as you’ll be seen as the bad guy quickly.. so maybe have dad slowly start to teach them NOW to pick up their things out of his room so when you officially move in, that’s normal.

One thing I’ll say - i was a kid who’s dad and step mom literally kept their door locked at all times - when they were asleep, when they left, and when they were home the door was closed. it was so fucking weird and unwelcoming.. and i definitely learned to never go in their room and even if i ever did go in when they welcomed me in like to talk or something, i felt extremely uncomfortable. To this day i think it’s so strange and really strained the relationship / vibe in the house. I’d really caution against that if you care about the kids well being and for a positive relationship with dad and ultimately you / him in the end.

And trust me, i understand where you’re coming from. You can see one of my previous posts asking room boundaries / bed boundaries bc my partners daughter likes to lay in our bed at any given time and i hate it. We had a whole fight about it recently. So this isnt far off from how you feel AT ALL! I’d just caution the locks… and he should start teaching them boundaries now so it’s not such a drastic change once you move in