Once they become juveniles you should back off on the shrimp and only give about once a month as a treat.
Good read for pellets and plant matter :
http://www.redearslider.com/index_nutrition.htmlThe head or 2/3's head rule works at any age for a happy and healthy turtle. I have used it for decades. As you are finding out they always beg and only get better at it with age. Always better to under feed than over feed which will lead to healthy problems.
You doing well with them !