My 9 week old twin boys are sleeping in their rock and plays at night (swaddled) and are sleeping about 6-7 hours at a time which is great. I don't put them in the RNP's at any other time except bed time. During the day they usually nap in their swings. Over the past few days, I have been trying to get them to nap in their crib or pack and play to try to get them used to it for when I start transitioning to the crib in a few weeks. When I put them in the crib/PNP, I put them together. The only way they will sleep in there is on their belly, but even on their belly they only sleep a few minutes at a time. After a few minutes, it's like they get startled and wake up crying. I think it may be because it's just open space that they aren't used to.
Any tips on how to transition them to their cribs?
Re: Sleeping in crib
First off, swaddle them when putting them in the crib ( I am assuming they aren't swaddled since you put them on their stomachs). Also, white noise is key for DD. Some people have also suggested warming the crib up with a heating pad and removing it right before you put them down.
Secondly, realize that daytime sleep and night sleep are run by different parts of the brain. While they may start napping in the crib, that may not correlate to sleeping in there at night. DS would nap in his crib, but wanted nothing to do with it at night until he was about 5-6 months old. DD naps in her swing, but won't nap in her crib even though she will give me 3-5 hour stretches in the crib at night. It's weird.
You also may want to look at something like the Magic Sleep Suit. I am getting one for DD to transition her out of swaddling and get longer night stretches. They look silly and seem gimmick-y but a couple of people I know personally swear by them and they get really good reviews online.