For greens, just put them in the tank and let them float (if you don't have a clip). I replace daily, but usually nothing is left.
For carrots, I like to use a peeler and make thin peels that float in the water like worms.
They need to eat in the water, and that's where food should be put.
If they're small, I'd try red leaf lettuce rather than mustard leaves. I've got two Eastern painteds that are a little over an inch in length who love to nibble on both tender dandelion leaves and red leaf lettuce.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-