You can alter children's costumes but if they have arm holes you have to take into account that your dog is about twice the back of a child and half the chest, does that make sense?
I'm a fan of homemade costumes. Here are two I made:
A kid's pumpkin costume is probably pretty easy as there's lots of wiggle room. Just don't forget the traditional black turtleneck, that'll knock 'em dead!