Do you think giving kids sweets as a reward kung magStop sila mag-tantrum is an effective way to discipline them? Or does it count as spoiling them?

It depends on the number of instances ng pagbibigay mo ng sweets as a reward. And yes, big part din yung if you are explaining why you are giving them the reward, kasi if not, it might send a different signal to them and they may count it as if they can just throw tantrums anytime in order to get what they want and in that instance, the sweets.