"They have never eaten a deep-fried dough stick."
If Duolingo uses 'never' in this sentence, it should be 你们从来都没有吗过油条 or at least 从没有. Otherwise it would just be 'They haven't eaten a deep-fried dough stick'
I assume you wanted to write 吃过 ;)
I’d argue that you need the “never” to translate the -过. If you just use “not/-n’t” instead, it doesn’t sound like you’re talking about (the lack of) a past experience, but just an instance, which would be 你们都没有吃油条。