You have 5 days from the date of biometrics for the package to be sent to Sheffield. If your husband is returning ASAP after the kids have their biometrics, then that's fine to send them from the UK. DO NOT SEND THE DOCUMENTATION SEPERATELY. If it will be more than 5 days after biometrics that your husband returns, you should OVERNIGHT the documents that are in the UK to your husband in the US using FedEx, DHL, or UPS (not Royal Mail or Parcel Force).
You only need one shipping label.