I am a Nanny and am wondering what to charge for an overnight.
Hi. It's great that you're staying overnight so the parents can go away! This is definitely something parents should consider, as long as you're comfortable sleeping at their house.
As for payment, most nannies tend to charge normal hourly rate for the hours the children are awake and then go down three to five dollars for the hours that the children are sleeping. I tend to think that is very fair, since you are just sitting and not keeping the children entertained.
If your family thinks it's too high, and this is a service you want to continue doing for them, you could always negotiate a flat-fee that works for both of you. It might help them feel more comfortable going away (and giving you more money!) more often.