What is the linking verb in where are the children?

In the sentence, there is no linking verb.

The easy way to recognize a linking verb is that a linking verb acts as an equals sign, the object is a different form of the subject (Mary is my sister. Mary=sister); or the subject becomes the object (My feet got wet. feet->wet).

In the sentence, 'Where are the children?' (the children are where), the place where the children are is not another form of the subject children.