When they started trying to climb into my arms each time I opened the cage, I knew it was time to let them out a bit. I put up plastic panels around the cage and let them out to stretch their legs - run around a bit a couple times each day. When they started to scale the cage to get over the pen, I let them explore the house a little as they were coming to hand. They did well and would go back to the cage and sleep in the hammocks there, and as my two had accepted them, I let them stay out.
I would sit down on the floor and they would all climb into my lap.
With kittens this little, I'm always very careful where I put my feet when walking around the house and friends who come over to pet kittens also know the drill.
They just weren't eating well and remained fluff, skin and bones after a week, so I bought some goat milk for them at the $$$ pet store and asked for raw food samples. It went over okay. But the moment I put the raw food down, they turned into the obligate carnivores, the little hunter predators they are. They growled. They snarled. The little girl grabbed a mouthful and ran off to eat it privately. Her twin brother covered the dish with his body and growled at his other brother - the grey boy. I could not defrost another sample fast enough! They gorged themselves the first three days. Eating as much as 1oz each in a sitting, then asking for more not 4 hours later.
I've bought the $$$ raw food now, and they get 2oz mixed in with their regular kitten wet food every morning and evening, with dry kibble to hold them over during the day and overnight.