If it's a client you want to work with and this is part of the client's requirements then yes, you should allow this or politely decline the business. The client has the right to ask for anything and you have the right to approve or decline. I ran an offshore software services company in Costa Rica after being a Division Manager at Cognizant and now I advise and work with 50 software companies in 25 countries and this is a common request that is complied with. This isn't to say you shouldn't make the reasonable argument that it is preferred that interviews are conducted just with team leads who will then vet and manage the rest of the team. Even though you recruited, vetted, hired and trained your employees, the client wants to validate that your standards and definitions of capabilities are in line with theirs. That is a reasonable exercise assuming you want the business and you feel that this investment of time and validation will lead to a long, fruitful relationship.