Basicly dont feed pellets everyday (if your turtle is over 4 inches), instead feed every other day, or two times a week. For a turtle under 4 inches, feed pellets everyday, just ONE time a day (in the morning) You can offer as much vegetables as they would eat, no matter how big or small your turtles are, you cant get fat off of salad.
I see your turtles are 2 inches, i'd say 3 - 4 pellets (if your using something like reptomin) is enough..that's how much I fed my turtles when they were that small. The how much food will fit inside their head thing is kind of confusing, i know..

No such thing exists for veggies though. I feed mine veggies in the morning, afternoon, and a couple of hours before the lights go off and they get pellets every other day (6 pellets, and when I'm using jurassi diet, half a teaspoon since they are realy small pellets).