Mine is 8 and so is her friend. For the most part we had fun and I am glad that we brought her friend. But my DH told DD that she wouldn't be bringing a friend along again until she was 18.
No, no, no - don't do it!!! I I feel for you - we took our 17yo DD and her 18yo friend 2 years ago. NEVER AGAIN! It was absolutely horrible. By day 3 I think they weren't even talking. DH and I hardly knew this girl - it was so incredibly awkward for the rest of us I couldn't wait to go home. Longest 8 days of my life - LOL! (Well, NOW I can laugh). I told her - no more friends!