Is anger or getting quicker temper a common side effect on children taking methylin for adhd?

Yes. Both methylphenidate and amphetamine medications can cause children to be irritable and short tempered. If this is a significant problem, speak to your pediatrician or treater about alternative medications.
Maybe. Medication for behavior should balance good effects against negative effects. Quick impulsive temper could be a side effect or not enough medication to capture symptoms. Review your concerns with your MD. If the symptoms are more in the afternoon make sure child is getting enough protein at lunch. They often don't feel hungry and this can lead to low blood sugar which can cause irritability.
I would not say. Common. And perhaps this is a clue that this is not the right medicine or dose. Talk it over with the doctor!