If you have good reason to think the children might not come back, you could make them wards of court. However, this is a very serious business, and can be awfully expensive. If they become wards of court, their names are listed at every port and airport in case of abduction. Only the judge in the appropriate court can then give permission for them to go abroad.
But, as I said, special circumstances apply here. Such as if an abduction or secret removal has been attempted before.