What the website you are talking about is doing, is including an element of skill. By spotting the ball, there is an element of skill.
By asking a question, there is an element of skill.
The questions need to be more than derisory so, you could not ask a question such as, “how much is 1+1?”
What you have to do is make sure that it’s not just a game of chance.
In all honesty, I think that spot the ball competitions are nothing about skill but are merely chance but that’s the way they are looked at.
By including skill, and removing chance, it becomes a competition and not gambling or a lottery
If you tell me exactly how you’re going to run yours, I can advise you further.