Ser is not defined for use with permanent things, and estar is not defined for use with non-permanent things. (I really wish Spanish teachers would stop teaching that myth).
Estar is used for states, which may or may not be permanent (él está muerto is fairly permanent, but is contrasted with él está vivo) but ser is used for essential attributes, which likewise may or may not be permanent (él es joven). Ser is also used with nouns as the object, which estar rejects (estar, on the other hand, allows adverbs as the manner of being: estar bien that ser rejects).
In your example, the predicate nominative is travelers, which is a noun, and so by definition to render your English sentence into Spanish, only ser can be used. Note, however, that using estar is perfectly grammatically okay in this example because viajero is also an adjectiv, but would not be a correct translation, as in English it would mean something akin to Y'all are on a trip? or Y'all are traveling? (estar viajero = estar de viaje)