How much should I as a nanny charge for a 2 year old and 2 new born twins?

I am a private nanny for a 2 year old girl (not fully potty trained) and they are expecting twins in August. I know an infant is a lot harder and worth a lot more, but with two infants the amount I know it's worth is too much to ask from one family. I currently make 11 dollars an hour for the 2 year old but I also do laundry and dishes and I don't have ANY benefits. Is doubling my pay too much to ask from them, or will I still be low balling myself? ( We are also going to work out some benefits so a lower pay might even itself out ) HELP!