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?

If you use it as a reward, I guess it's totally fine. Rewarding is different from bribing. And yes, I agree that whenever you give a reward, it should be clear to them that they're getting something because they are doing something good. It's just like giving a star when they do good in school. You don't do it too often siguro para they would still get excited if there's really a need for you to do that pag medyo excessive na ang tantrums.