Is « πολλή» the feminine form of «πολύς (πολλή πολύ) meaning a lot of? As it has the three genders, I suppose it is an adjective. And « πολύ» here is wrong because it is an adverb?
Exactly. In nominative, it's πολύς (m), πολλή (f) and πολύ (n). When it appears as an adverb, it's πολύ.
In this sentence, energy is being described, not a verb, so we use πολλή, given that ενέργεια is feminine.
My idiomatic translation, "the kid's got a lot of energy," was rejected outright. Possible to include it?