Invite addressing

Pburke2010

I have two questions to address invites. 

1)  For kids, its it:

John and Jane Smith

Pamela and Spencer 

(Age order?)

2)  I have two married couples living in the same household, with another cousin (who is single).  Do I really need to send three invitations to the same house??? Or could I do:

John and Jane Smith

Mike and Mary Collins

Joseph Jones

Thanks!!

coraleetbwed

Outer envelope should be addressed
Mr. and Mrs. John Smith

Inner Envelope is
Mr. and Mrs. Smith
Pamela and Spencer (older to younger)