I have kids, and my experience perfectly matches the empirics on this one: There is no such thing as a sugar high. You may just have a wrong perfection of the "normal" state of you kid. The "hyperactive" version of your kid is just the normal way your kid behaves. What you observe if the blood sugar level of your kid wanes is fatigue, nor calmness.

Edit: I can also see this with aquaintances who also believe in sugar highs: They only report this for the sugars perceived as worse than the "healthy" sugar sources, e.g. they project a sugar high if the kid gets a piece of cake, but not if it drinks 300ml of undiluted apple juice.

I think you might be selectively sampling things. I.e. you notice when your toddler is hyperactive after eating fruits but not when he or she is hyperactive after eating other foods.

When it comes to a sugar rush, potatoes, rice, and corn have a significantly higher glycemic index than oranges, apples, and mangos (potatoes are about twice as high). This means that potatoes and corn will more quickly spike your toddler's blood sugar levels quicker and to higher levels. Does your toddler get more hyperactive after eating potatoes or rice or corn?

I*ve seen kids bounce off the walls because they were going to watch their favorite movie, too. Or get to play a game. Or... Well, anything, really.

Isn't bouncing off walls just a thing kids do?

That's hogwash. They are just kids.