Do we say,Where is or (are) the boy and the girl?
I assume your question is "Do we say "Where is the boy and the girl?" or "Where are the boy and the girl?"
What do you think the answer is? If there are a boy and a girl, how many children are there? That answer should tell you whether to use the singular or the plural.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.