We kept our last new kitten (she's almost one now) in a spare bedroom devoid of anything of real interest other than cat toys until she was about 3 1/2 months old. Her segration had more to do with our adult cat's relunctance to accept her and less to do with her being a kitten, however. She was SO ready to get out of there by the time we decided it was time. From there, we limited her access to our unfinished basement where the cat boxes normally are because she was just too little to really navigate the stairs. In time, she was given access to the basement and the first floor of the house. The cats are normally restricted from going upstairs to the bedrooms, per my DH's strong preference to sleep cat-free. Fortunately for him, there's a door to the upstairs.
"Their" part of the house is pretty well kitten- and cat-proofed. I've had some success using a spray bottle of water to keep them away from, off and out of certain things, but if something is really important to us or could injure or make them sick, we have moved it from their territory or out of their reach. It's just not worth the risk. Thankfully, our adult cat has never been one to really get into much of anything. My now-deceased cat and our current kitten, however, couldn't/can't be trusted, especially with plants. Perhaps because I've had cats for a while, I sort of have an eye for what they might get into so I can better anticipate it, but if were in your shoes, I'd pay close attention to anything they could eat or chew. Beyond that, anything you just don't want them to get into, e.g., a trash can, make sure to put away or close off.
Be careful when you're getting into drawers or closets. I've locked all of the cats into things at one time or another. One of my cats spent a long day in a drawer full of cycling gear once. Bless her heart; all she did was make a bed for herself and fall asleep. Another of my cats spent a day in a linen closet. Somehow or another, he managed to climb to one of the higher shelves and knocked all our meds down.
So, my basic advice is this: Let them free once you feel like the house is largely "secure." IME, kittens survive a lot more in the way of mishaps than you think they would. Given them plenty of cat-friendly diversions and hope for the best! Remember that they sleep a good part of the day anyway so make sure they have blankets and towels to lay on if you'd like to keep some of your furniture relatively hair free.