Let them work on filling the medium, when they are a frame or a frame and a half you could put the next brood box on. I would leave the brood where they are, may move a couple of the honey frames up to 2nd box. Good luck with your new bees.
I have moved the outside frames in one on the sides if they aren't messing with them, or in other words, switch places with outside and next frames when inside frames are being worked.
Joe