I've noticed my hens aren't wild about celery...some will peck at it if they are really bored but if there are better treats around celery gets ignored.
7.5 weeks...yours are still pretty young...usually they get more bold about trying new things as they get older. Are you just throwing the veggies in there whole? Are you chopping them up? I bet they would try the softer things like grapes and tomatoes if you gave them little bite-sized pieces at first. Once they learn those things are food then you can throw them in there and they will attack...
Lets see....treats my chicks and younger birds have liked.......birdseed or scratch grains, grass, leftover scrambled eggs, leftover cooked rice (they LOVE that!), etc. Younger birds seem to like smaller treats at first....they'll tackle the big stuff when they get older... and wiser