"Where is the kitchen?"
Translation:Onde é a cozinha?
From what I know as a learner, both have the same meaning. "Cadê" tends to be more informal and you might this use this one more in spoken form. However, informal or proper written Portuguese you would have to use "onde". Keep in mind when using "Cadê", you have to use an article with it. For example: Where is the kitchen? you would say " Cadê a cozinha?"