Smurfs have been with us for so long that almost all of us watched them as a kid and fortunately they haven't changed much in all these years.
Their main color combination is white and blue, except Papa Smurf is dressed in white and red. And that's basically it.
Image by skpy, CC BY-SA 2.0, Flickr
Kids can wear white pants or jumpsuit with white shoes and blue T-shirt. You can add white gloves too if you want. And you can add some blue Smurf make-up and the costume is ready.
And don't forget to put blue make-up on kids neck too.
Girls can wear white dress or a skirt with blue T-shirt. And still one thing - Smurfette has long blond (yellow) hair.
You can find quite a lot of pictures of Smurf costumes on Flickr and Pinterest - check them out and they may give you an idea or two.
Image by Pippi Dust, Public Domain, Openclipart