"You are small children."
Translation:Jste malé děti.
Very often personal pronouns can be left out. In most cases, they are more or less "optional." But they can be used for emphasis, for contrast, or to add clarity.
As you can see from the earlier discussion, both "vy jste" and simple "jste" are accepted in this exercise; most exercises will accept answers with and without the pronoun. (Please always read the existing discussion before posting.)