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.