A. Is everyone finished brushing teeth?
B. Is everyone finished brushing?
In asking children if they are finished brushing their teeth.
Which sounds natural to native speakers?
Or is there more natural way of asking?
Note that all the people saying "have/has" are BrE speakers, who consider have/has finished to be the verb in that sentence. In AmE it is common to use finished as an adjective, in which case is would be the correct verb.