First three sentences of a short story that's supposed to be for children ages 9-14.
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Leo scrambled backwards, almost tripping before managing to right himself. His mouth fell open at the sight of the lake bubbling and glowing.
The head of a snake popped out of the water’s surface, its bright red eyes flashing.
I highlighted all those "ings" so you can see just how many there are and how often you've used the same sentence structure in just these three lines.
I feel like the beat of Leo recoiling from the lake goes on too long. The near-miss action of "almost tripping" and "managing to right himself" deflates the tension. Tighten that up, add the action of the next sentence, and you've already got a tighter opening, e.g. "Leo stumbled backward, mouth open at the sight of the bubbling, glowing lake."
I think you've got an interesting scenario here, but the flabby pace would put me off reading further.