"Where will they go tomorrow?"
Translation:Куди вони будуть їхати завтра?
No, it has to be іти or їздити since they are the definite variants (in contrast to ходити and їхати). The indefinite variants are exclusively used for habitual and repeating movements (often including not only the way to something but also the way back). When a question is asked about where someone is going to, the definite variants are usually used, since you want to know where exactly there a going to now and not where they are going to now and also every other day.