If they are dual Citizens then I don't think it's mandatory for them to leave on US passport, but you should check with State Department, USCIS , an immigrationj attorney or call British Embassy in US and ask if they know.
What matters is :
1. You must have full custody and right to take them with you.
2. Thay must have valid document to leave the country.
Good luck,
IE
Posts: 2501 | Location: NJ, USA | Registered: 03-11-2006
You can carry both of them. When you enter UK show them your UK passports & on your way back do vice versa. Carrying both passports is not a big job as they only weigh few grams each.
Elizabeth, if you or anyone has a dual citizenship,it does not matter one bit,with what passport you leave the country.Because your passport is only being used as an ID.Thats it.
We really don't have time to get both and I keep getting word that they have to leave on US passports like there might be a problem at the airport when i take them out or something! Also If there are no i-94 forms to remove what do they do?