H.F.
Is there any way that their Dad could drop them off in the mornings instead of you? Or could you carpool with another mom, she does the drop offs and you do the pick ups. It may be that they will behave better for someone other than mom. Another idea would be to have consequences for their behavior, like if you throw a tantrum in the mornings you get no cartoons after school (or no dessert that evening or you take away their favorite toy etc.) but if they behave each morning they get a sticker on their chart that you make them and when they have filled up five spaces on the chart with stickers (for five tantrum-free mornings) they get a special treat like a toy from the dollar store or an ice cream cone, whatever motivates them and is not too expencive for you. Good luck!