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My middle-grade novel is third person with one primary and one secondary viewpoint character. Sometimes the narrator hovers a bit more, sometimes the narrator is more in the character's head.

The narrator primarily describes things the way the character might. For example one character will refer to one of the younger children by name while the other viewpoint character calls him "that little twerp."

As these characters travel back in time and across the world, they encounter a lot of things they're not familiar with. Even things an educated adult from their time and place may not know.

Do I name them?

I don't want half the dialogue to be "oh what do you call that?" Nor do I want every description to be non-specific to their new setting. If there's an English modern version of the thing they're describing, I use that name. For example, one character talks about eating cheese that looks like feta and wearing a tunic, even though the local names are different (I'm not talking about translation here, I mean the correct names in English, which can be a loan word or not).

OTOH, I've also referred to the enormous tree in their courtyard as the terebinth tree (only when they arrive, after that it's just a big tree) and I name the cloth in use as linen, a fabric used in modern times too, but one the kids might not know.

Where is the balance between allowing the narrator to describe the scene and keeping the point of view?

Describe the scene from a person's point of view.

You say this:

these characters travel back in time and across the world

If I were to travel back in time and across the world, then I would use vocabulary that I know. If something looks like a hole in the ground to me, then I'll describe it as a "hole in the ground". If people defecate in this thing, then I'll call it a "toilet". It may not look like the comfy, flushy toilet that you sit on, but it is used like a toilet. So, I'm going to call it a toilet.

In a Western-style bakery, breads are baked in the oven. According to my aunt, a native in China, apartments don't necessarily have ovens, because, well, people don't usually bake goods. But more and more people are buying ovens, because they want to make Western-style baked goods, like cookies and cakes. But Western-style desserts in China are more - how do I translate this? - 细腻. The dictionary translates this as "detailed, meticulous, exquisite". The feeling feels comfortable in the mouth, not too gritty and crumby. Meanwhile, my aunt would say that Western-style baked goods in America are more 粗糙, and the dictionary translates this as "crude, rough, coarse, gruff". She adds that American baked goods are far too sweet for her taste, not very palatable. But then, that may be because she comes from an older generation, and she is not accustomed to Western food.

As you can see, my aunt is clearly describing everything in her words, from her point of view, from her own personal experience. The only way I can identify with what she is experiencing is if I try it for myself. When I try something, the sensations will become imprinted in my memories, and those memories will become attached to words; then, in the future, I will use the words to describe such experiences. These words may or may not be translatable across languages. 腻 can be used as a verb in Chinese, yet it is used to describe oily foods and glutinous rice-based foods that when eaten will give you a sickening feeling. You can find definitions for it in the dictionary, but I learned this word contextually by personal experience, which gave this word an emotional meaning. I suppose if I want to describe the same feeling in English, then I would say, "It's too oily for me!" or maybe "Too much oil! I feel sick!"

With that said, a narrator can probably write about how something makes the narrator feel in the narrator's native tongue. Explaining in the narrator's own language gives the narrator a voice.

There is an alternative that I see to the proposed answers. You can in fact use the proper names of items. Here's how.

The first time the children encounter something they are unfamiliar with, they might ask what it is, or your narrator might go

they did not know it at the time, but found out later this cheese [or whatever] was called [whatever it's called].

After you've done this once, you've established a trend. The reader can now assume that asking about the names of things / finding out later happens offscreen for all instances the children encounter items they are not familiar with.

This approach necessarily means your narrator is slightly further from the MCs' mindset. With the second option in particular, the narrator is looking back on things that happened to the MCs, rather than being with them in the moment. It is, however, an option that you might want to explore.

Kids tend to give things nick-names. All you really have to do is describe it, and give a it a nick name. After that the reader will remember what you're talking about.