1. If they are under 1 year old, sleep them in their infant car seat. They usually love them anyways, it is only for a night or two and they are really warm. Especially if you have a Bundle Me (http://jjcolecollections.com/original-bundleme) in there with them. And then you don’t have to pack a pac-n-play.
2. Depending on how cold it is at night I usually layer as follows – onesie, light weight long johns (we have REI brand), fleece pants and top, long sleeve sleep sac, sleeveless sleep sac, down coat/vest, hat… and yes, then they cannot really move. And this is the most layering we have done – probably 40 degrees at night. I would probably just use sleep sacs and a hat- with a tighter fitting long john or sleep n’ play underneath if it was 60ishdegrees. We have a lightweight Patagonia hooded long john top (pictured) that I LOVE because I can pull the hood up to keep Esme’s head warm at nights, yet it isn’t so bulky that she fights it or has it pull down over her face while she sleeps.
4. You can test how cold/hot they are by feeling the back of her neck, that is how I usually check throughout the night, cause it will be coldest 3-5am.
5. Don’t layer them up too early, they’ll sweat, then get really cold. We’ve made that mistake. Often, when I wake up to pee at 2am, I add a sleep sac or hat before I go back to bed.