If you want to avoid having them eat long blades, the kids can either tear up the grass to small pieces and feed those, or hold onto the end of the grass and allow the chickens to tear off pieces themselves. I find either way works.
Long blades can definitely be a problem in the crop. If they can't bite off pieces (like they would when foraging), they will push the blades down the esophagus with their tongues.
Additionally, grass isn't one of the chicken's omnivore choices for greens. Chickens are foragers, not grazers. Much more appropriate for them are tender forbs like clover, peas, radish, buckwheat, etc..
Grazers like cattle, horses and sheep, evolved to eat grasses (monocots). Browsers and foragers eat dicots. Plants that emerge from the seed with 2 leaves.
Goats and deer are browsers.
Chickens, like many other birds and mammals like rodents and humans are omnivorous foragers. None of which do well on grasses.