So I've got this family with two adults and some children. I want parts of their house to be inaccessible for the children, but if I use the "Lock" option and choose to only let the two adults in, that means that non-playable sims can't enter either. This is a problem, because I want guests, the maid etc. to be able to enter, but I can't do this while also keeping the kids out. Is there any way I can do this, or perhaps a mod I can get to enable such filters? (Preferably not age-filters, I'd like to be able to invite children/teenagers as guests, but if that's the only solution then I suppose it would be okay)

Chain_Reaction over at NRaas has added a locking system to GO Here - In Testing. There are some tips etc On This Page
Many of us have been using the mod since it went into testing and it's really doing an awesome job.

The Barrier To Entry System at the Store nitromon mentioned has other specialized options you might want to check out.

Thank you! I got that version of GoHere (already had the "normal" version so I had to replace it) and it seems to work. I can choose which sims can and cannot open certain doors, so that's a mission accomplished! However, the "can't open" list gets messed up sometimes and adds a bunch of Sims, so that guests and Bonehilda can't enter. I'm not sure if the mod had issues or if I'm doing something wrong, but honestly it's not a big problem, I can easily fix the lists anyway. Thank god for NRaas, their mods are the best!!

Glad it is working for you Kithie. I do not think the mod has issues OR that you are doing something wrong. I have found that when a household changes the listings are also updated. For example a new baby aged up to Toddler. I have to go in and move the sim to the Cannot open the door. Maids and babysitters often need to be added. It takes a little time to get used to some of the tricks but I find it overall more useful than not and as time goes on with it in testing Chain might tweak some of the options.