X.O.
This is the first time I've heard of a "sleep under," and it sounds cute, but to differentiate from a regular birthday party, I'd invite the guests to come in their pajamas. (Also makes it easier for their parents to get them to bed when they get home if they are already READY for bed).
If you are planning on it being for 2 hrs, would it be something like 5-7, or 6-8? Are you feeding them dinner? Or an evening snack? Everyone eats dinner at a diferent time and has different bedtimes, so you might want to think about that aspect that some kids will be hungry and cake/ice cream won't be sufficient.
Maybe have them help make a snack to eat that DOESN'T require cooking.
ETA: Maybe buy a bunch of plain colored pillows and let the kids decorate them? Read them some bedtime stories. My 5 year old son LOVES "Amelia Bedilia," which I was so excited to introduce him to.
ETA2: Get some very small plain wooden trinket boxes (with a latch) from a craft store & have the girls decorate them.