If you live in a colder climate, buy your child's costume a size or two larger so s/he can wear sweats or a jacket underneath without ruining the effect.
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As a child, I lived in rural Illinois. October there is cold. Sometimes it snows! The flimsy store-bought costumes leave everyone shivering. A warm costume is worth the effort.
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I need some help with ideas for a Halloween costume for a 1 1/2 year old. I looked at Walmart but the costumes for that age are just so boring and standard.