I've been pretty convinced of the child theory too because of the stool in particular, but it kind of makes sense that Bella might have turned that room into some kind of weird James shrine or refugium or whatever (I assume that's some kind of technical term I don't know).
I really need to replay Bella's path but I seem to recall from the last time these discussions came up that Jill was surprised to hear from the MC that Bella had locked that door. It's kind of impossible for me to believe that Jill wouldn't know about Bella having a kid that died, and if that was the kid's room, why would she be surprised that Bella is keeping it off limits? It might be more likely that before Bella locked it, it was just a normal room, that Bella has transformed into something more sinister. That is also consistent with what some people believe we've seen under the door, i.e. the boots. There could be a mannequin in there that Bella has set up to dress in James' clothes or something equally crazy.
Also, just as a small thought, I always found it a bit unlikely that the kid's room would be on the ground floor. Usually you'll put your kid's room the furthest from the entry of the house as possible in case an intruder comes in.
Ultimately though none of this accounts for the foot stool so I dunno
