Ok here’s the skinny…
I have twin daughters, 9 years old. They have a little white fluffy dog.
Thye’d like to go out some sort of themed costume for halloween with the dog in costume as well.
Any ideas. (nothing dirty please…these are my kids…)
Two clowns with a circus dog.
Dress the girls up like dogs, and have them pull the dog in a wagon (like they’re sled dogs).
You could do a Wizard of Oz thing if they don’t mind having the dog the wrong color.
Two shepherds with a sheep.
One could be the Grinch, the other a Who and the dog could be the Grinch’s mutt.
Dress one girl as a graham cracker, the other as a Hershey bar, and call them all a S’More!
I love Scarlett67’s idea!
Dress them up as atoms, make the little dog hydrogen. 
No no, the dog is oxygen, the girls are hydrogen.
Dress the girls like complementary halves of a hot dog bun, and put a mustard and/or ketchup costume on the dog.
Dress the girls like sheep, and the dog like a sheepdog.
Or perhaps the dog is carbon, or silicon, and the girls are oxygen.
Or the dog is a helium nucleus, and the girls are electrons.
Or the dog is a lithium nucleus, and the girls are again electrons.
Ooh, ooh, the dog is Mars, and the girls are Deimos and Phobos.
Or, if you don’t like totally geeky stuff like that, you could have the girls be chocolate cookies to make an Oreo.
You could have one daughter be a hunter, and the other daughter be a rabbit or fox, and the dog’s the huntin’ dog.
Or…
Put a horn on the dog (will he wear anything, do you think?), call him a unicorn, and the girls can be fairy princesses.
Or…
Paint some spots on the dog like a dalmation and the girls could be firefighters.
Well, I’m out.