Kamille sorry been gone a little , life's issues to take care of. Steve's veggie plan is very good to follow. Also a few let a leaf float in tank for them to snack on as needed. I have safe live plants for Piggley to snack on. Some people only feed stick food every day , not good ! Varity is say sticks , worms , crickets , shrimp , minnows , ... etc live and or dried. I use head size for the protein not veggie. Change it up sure you for yourself don't eat beans everyday. Sometimes I skip a day or two on the protein and its only veggies do to his size. For Piggley that's only protein two days a week but maybe a cricket here and there? When it's protein day may get two shrimp , four sticks , four mealworms , then say two crickets. Then he may get some live plants if he wants. Just an example I use / adjust to your turtles size. Piggley is older than yours and I have him about 80% veggie now with less protein , that's just me. If ever not sure better to under feed than over feed !!!
Look at some feeding list on this forum BUT also read if any comments and or recommendations that go with them.
Baby turtles need more protein due to they are growing at a fast rate. But as they get older growth slows down and need more veggies. Where most go wrong they feed an adult as a baby and that's BAD for health.
When I say veggie it's a mixture not just one veggie all the time. Good read here :
http://www.redearslider.com/plants.htmlAs far as how to change there diet just do it. They will adjust , turtles will always beg , in nature they have no idea where there next meal is coming from. Captivity its us to feed them health ! If they beg , and are the BEST at that , make up a game to play get there minds off food give them attention not food.