Well, hate to break it to you, but that's the $64,000 question right there.
The inability to describe a sensation to someone who has never experienced that sensation is one of the fundamental gaps in human language that we are incapable of overcoming. Though we have a word for this: Qualia!
Basically, a Qualia is a subjective experience and is difficult to explain in words what the difference is. She would not be able to describe the experience without putting it into a frame of reference to other experiences. The classic example is imagine you meet someone who is color-blind in such a way that he can only see in shades of grey. How do you describe to him what blue is? Well, the sky is blue... and the water is blue... but grass is not blue. But to this person, he sees blue as a shade of grey... the sky is grey. Water is grey... but so is the grass.
In short, there is no way to describe the experience. You could show him that mechanically, Blue falls within a certain wave length of the visible spectrum of light but... it's a shade of grey that's slightly less or more grey than the green and purple between it.
This isn't as out there as you think as there are quite a few languages that lack a word for a specific color. In Chinese based languages (Mandarin, Cantonese, Japanese) green is traditionally a shade of blue and not a separate and distinct color like it is in English. Their respected words for green are a relatively new invention and historical literature doesn't contain them and even today, some art still does this (Despite Traffic Lights using a distinct Green color the world over, even in Japan, you will often see Anime traffic lights with a Blue color. Having personally been to Japan, it's definitely a Green color in real life). Similarly, this is why the English Language word "Orange" does not rhyme with anything... it's a loan word derived from the Arabic word Naranj and didn't enter the English language until very late (a 1512 will is the first recorded instance of it). Prior to this, Orange was either a shade of red, a shade of yellow, Yellow-Saffron, or a blended shade depending on what it was closer yellow or closer to red (Red-Yellow or Yellow-Red, respectively). This is why people with bright natural Orange Hair are usually called "Red-Heads" unless you're the President (sorry, could not resist).
With taste, you have the same thing. She would only describe it in terms that she knows... it's red, like meet... it has a crunch, like a bone from meat... but it's not as hard as a bone... it's softer... like a mushroom... but it did not taste like a mushroom... it was more like a leaf that came right from the tree... not dried, but still wet... but the taste was so much better than any of those things (fun fact: Human taste likely developed to tell us what was safe to eat... we would prefer sweet to bitter because bitter tastes use alkalides in their chemical make up and alkalides are usually poisonous to humans... but human milk is sweet, which encourages children to look for sweet things, which tend to have sugars which we need for energy... Better living through Chemistry, amiright?).
She should have name for it that she'll call this wonderful thing until she gets told what it is (I love the idea of calling it Mushmeat, but you do what you like).
If you'd like to see other writers deal with this problem, The Giver has a segment where the hero thinks there is something wrong with him because something happens to his apple and people's faces and he doesn't know what the hell it is and no one else sees it (turns out that humans in his society were bred to not see in color... those that still can are something special). The book series Animorphs does a lot of different sensory descriptions as the heroes all have the power to change into animals and usually there are good lengthy descriptions of the new sensations they couldn't have known til now AND has aliens that don't have great senses but are brain parasites. All books feature the animal morphing save for a few prequel titles (I think two feature characters that don't morph, but all feature characters that can change senses in some way). Book 6, book 29, and the novels The Hork-Barjir Chronicles and Visser feature the parasite aliens describing various host species for the first time (Sorry for not giving the title of the books... they always followed the formula of "The [Insert One word that rarely had anything to do with the story here]" formula and there are 54 main title books. I'm only human!). Any book with the characters of Ax (easy to identify... they all have a blue centaur thing with no mouth on the cover...) are basically the author's excuse to really play with sensory descriptions down... naturally, having no mouth, Ax has no concept of taste... and becomes the biggest foodie you could ever imagine... and in some cases, the fact that something is not food is not an excuse for him to not love it like it is (His first book originally shipped with a promotional bookmark that "he" said was a gift to the reader... and had a bite like rip in the upper corner because he couldn't resist). He also has a fascination with sounds, which he also doesn't do ordinarily due to the lack of a mouth (his race basically communicates via a limited telepathy). Finally, The Andalite Chronicles has this conundrum discussed in depth (pun not intended) when one character explains the nature of a god-like entity to be akin to trying describe the concept of "height" to a hypothetical species that exists in only two-dimensions of space length and width... and later briefly demonstrating this. Suffice to say, every book in the series will feature someone experiencing a sensation that is heightened, diminished, or completely alien and new to them. Also, for a late elementary school target audience, they're very good reads.
