Here is a list of 40 great family costume ideas that I found on various blogs and on Pinterest that you can do together.
Dressing up at Halloween is one of my favorite things to do. Getting the entire family involved with a theme is even better. We've only done one of these on the list but now that we have 4 members of the family we have more options. This list includes great ideas for any size family!
Pick a decade! A few years ago we went as 20's style flapper
Everybody loves Sock Monkey's
The queen of England and her Royal Guards
The Sesame Street gang costumes
Madagascar Crew
Teenage Mutant Ninja Turtles. Have a few extra members of the family? You can go as Splinter (the rat) or April as well.
Napoleon Dynamite family – Vote For Pedro Shirt
Disney's Up
Despicable Me characters. Don't forget your Minions either!
The Hunger Games
The family that does Laundry together at Halloween, stays together!
Mary Poppins is always a hit! (English Nanny, Chimney Sweep)
Don't get stranded on an island! Gilligan's Island
Doctor Who! Doctor Professor Costume
Bring on the Lobsterfest. Dog rain coat
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Rock on with KISS
Where the Wild Things Are, more pictures here. 
Aladdin costumes from Our Life is Beautiful
Many characters to choose from in the Toy Story movies
Tori Spelling and her family dressed in their Alice in Wonderland costumes (Credit: Michael Simon/startraksphoto.com)
You can't do Cat in the Hat without the fish! Vital part of the story. 
The Jetsons
The Incredibles Costume
Your classic BLT, but much cuter.
This M&M family is super easy to make and also very cute. 
Willy Wonka and the Chocolate Factory, what an adorable Oompa Loompa
Super Mario Brothers. Always fun! By Givers Log
Disney's Phineas and Ferb
The Muppets
The Avengers costumes
The Flintstones
Mystery Incorporated. Scooby Doo and the Gang!
Star Wars, by Bird on a Cake (lots of other great family ideas on her blog)
The Wizard of Oz
Crayons Color family
This pirate family turned their red wagon into an amazing ship! You can also make little mermaids and parrots to go with your Pirate theme. Great Peter Pan idea too, with indians, Tinker Bell and lost boys.
For the bee lovers! Bee keeper costume. 
Harry Potter by Sara Jane Sews (more pictures at blog)
Popeye the Sailor Man, (you could also dress up another little one in spinach!) by The Patriotic Peacock
The Addams Family– The perfect Halloween family!
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Wow what great ideas! Every family should be able to find a theme that works from that list!
Those were all so awesome, of course, my favorite is the Doctor Who family. I wonder if I could talk my own children into dressing up as Daleks??? Maybe a Weeping Angel?
That Sock Monkey Family is adorable!
It’s never too early to think ahead. I love these ideas, especially the sock monkeys!
Haha I love some of these!! We attempted a Yo Gabba theme last year but my son decided he would rather pass out candy. Therefore the costumes were an epic FAIL
I think that Up costume might be my favorite – it is way too cute!
Those are SUPER cute ideas! My favorite has to be Up! My husband doesn’t dress up so we haven’t done a family costume but next year I’ll have two kids to dress up so I may just have to do something with them.
The Despicable Me family costumes are great!
Thanks for including my family in this great round-up! I love the Napoleon Dynamite costumes…made me LOL! 🙂
Love the ideas here – they are so creative!
My son was crazy over “Up” a few years ago and I hadn’t once thought of making a house out of cardboard. He’d have loved it!
Very cool ideas! I am in awe of any parent willing to wear a teenage mutant ninja turtle costume for their kids!
Cute. Now that we have 2 kids I have been wanting to try and match their costumes but I wasn’t sure what would be cute. This gives me lots of ideas.
I should totally do The Little Mermaid for Halloween heheh
There are a lot of great ideas here! I think I would go for the Flintstones. Of course, I think my kids would prefer to me M&Ms… at least until they understand that they aren’t going to be covered in the ones you can eat.