Once you have all the relevant info, here are the basics.

Married couple, same last name:
Outer envelope:
Mr. and Mrs. Peter Pumpkineater
123 Main Street
Anytown, Alabama Zip
Inner env (if applicable): Mr. and Mrs. Pumpkineater

Same married couple with small children:
Outer envelope:
Mr. and Mrs. Peter Pumpkineater
123 Main Street
Anytown, Alabama Zip
Inner env (if applicable):
Mr. and Mrs. Pumpkineater
Ruth and Samantha

Married couple, different last names OR unmarried, cohabitating couple:
Outer envelope:
Ms. Catherine Douglas
Mr. George Hendricks
456 Short Avenue
Anytown, Alabama Zip
Inner envelope, if applicable:
Ms. Douglas
Mr. Hendricks

Again, add small children’s names to the inner envelope. Small children do not need to be addressed as Miss or Master (the rules for that are complicated anyway.)

Older children, say 15 and up, might greatly appreciate receiving their own invitations. They would be addressed as Miss or Mr.

Notes:
- Never-married women may use Miss or Ms., depending on their preference. If you are unsure and don’t want to ask, Ms. is safer.
- A widow may still use her husband’s name: "Mrs. Warren Nichols," even if Mr. Nichols is long deceased. If she has taken to going be her own name, however, she is Ms. Ramona Nichols. "Mrs." is never properly used with a woman’s own first name.

Hope this helps to get you started. Let me know if you have any further questions. Best wishes!
References :
Miss Manners’ Guide to Excruciatingly Correct Behavior

Mr. and Mrs. John Doe
123 Main Street
Atlanta, Georgia 12345

Then, on the inner envelope you include the names of all people invited, listing children in order from oldest to youngest:

Mr. and Mrs. Doe, Mary, James, and Thomas

If any of the children are aged 18 or older, they technically should receive their own invitation.

check out http://www.wedalert.com/content/articles/address_invitations.asp they have proper envelope wording to every situation.
References :
http://www.wedalert.com/content/articles/address_invitations.asp

It’s that simple. You do not need to address each child indvidually as with large families that would look silly and take up too much space.
References :

Mr. and Mrs. Smith and Family
References :

If they are a couple with no children it would be to:
Mr & Mrs. John Smith

If they are an unmarried couple at the same address it should be to:
Mr. John Smith & Ms. Jane Doe
References :

Mr. and Mrs. Joe Smith
Master Jim Smith (for an underage son- Miss for a daughter)
222 North Boulevard
Anytown, Mississippi 35660

In formal inviations you write out everything – no abbreviations
References :