I'm surprised no one else has mentioned this problem. I don't have apartment life but regardless I created an apartment for my sims game with many other expansion packs. I built various apartment rooms for different sims who live in the building. I have two families with babies in separate apartments, but for some strange reason when a mom autonomously takes her baby to feed her a bottle the foolish mom leaves her apartment with the baby in hand to try to enter another families apartment. She will always avoid the fridge in her own place to try to enter another families apartment to use their fridge. I already have a hack to stop sims from entering certain rooms but they still try to enter doors that are locked and keep stomping the floor in anger when they can't get in. They also try to get into other apartments I have in the building constantly avoiding going back to their apartment to use their own damn fridge. Is there a way to stop this madness?

Not really; without Apartment Life the game sees the rooms as all part of the same lot, so sims will choose to use whichever fridge they feel like at the time (based upon the fridge attractiveness/ proximity).

Learn to design your building around the routing protocols.

That's it, that's the whole secret. Since you don't have AL, you can't make the two units into separate lots, so the neighbor's fridge is advertising more strongly to the mother than her own, and/or is perceived by her to be closer than her own.

We can give you hints about some of the non-intuitive ways the routing protocols work, but since we don't have access to the source code, it's all extrapolation from experience. You cannot look at this problem from a humanocentric standpoint and hope to solve it, but must figure out, from observation and experiment, how your sims are experiencing their world. Sims prefer turning right rather than left, advertising on objects reaches through portals to throw them off course, a refrigerator on the floor immediately above their heads may seem closer than one in the same room with them.

In this particular case, the simplest solution would be to put a refrigerator in the nursery, right next to the crib. But depending on the layout, even that's not foolproof. Especially if Mom perceives her neighbor's fridge to produce better milk!

Without AL your apartment isn't an apartment just a really large house, so sims can go wherever they please. Now if you want to stop them altogether Simgogoical has some door locking mods. Or do what I do when I want a communal living area that isn't an apartment and that is to use the dorm doors. You will need 2 for a couple, each sim claims a door and then you lock or unlock as you want privacy. This works fine until you have kids then you either leave one door unlocked or add more doors.

Inge should totally rename her site to this.

Dang, you can tell when I just woke up.
Simlogical. :P