maybe not quite understanding. are you say that their business are the members of the LLC not them individually.
if so based on what you provided their businesses (sole proprietorships) are disregarded and the k-1s go to the owners which means you need the owners SSN
if there business were S-corps or partnerships then the k-1's would be issued in the name of the S-corp or partnership and the EIN would be used.
the issue is at their end. filing a schedule C doesn't allow for reporting k-1s from pass through entities (like your partnership). rather they would report as if the k-1 was received directly by them.
