Does sugar-intake really cause hyperactivity in children or it's myth?

No. I know of know legit study that backs this up.
Very different. Adhd attention deficit hyperactivity disorder is often caused or worsened by fluctuating blood sugars as a result of sweet drinks. Reactive hypoglycemia leads to inability to concentrate as the brain is temporarily starved for glucose. Then the low sugar levels stimulate the release of adrenaline leading to the hyperactivity.