In my long-ago comment below, I was surprised that my answer "a small child" was corrected to One small child. But now I see "a small child" is the answer or "one" answer or "an" answer.... It's often not clear whether one or "a" is called for in an isolated phrase or sentence starting with ett.
As you may know, adjectives conjugate according to the gender and number of the noun. Små and liten both mean small or little, but are used slightly differently. You use liten for singular en nouns (like en liten hund), but små is ALWAYS used for plural (Flera små hundar). Here is a link to a useful website explaining this.