Snack ideas needed

I run a daycare and all of the sudden one of the little girls I have been watching for 3 yrs doesn't like any of the snacks I offer her.

Graham crackers, granola bars, fruit cups, veggies and dip...she says she doesn't like them!

I do give her pretzels and cheese crackers, but that is all she will eat.

Her dad asked me to give her a snack before he picks her up so she doesn't want to eat before dinner. Any suggestions on some different snacks that I could try?
Have you tried celery stalks with peanut butter? You can also try to give her apple slices with peanut butter. If she likes cream cheese, there is the possibility of mixing shredded carrots and celery with it. Make it like a spread and put them on little whole wheat crackers.
Mini sandwiches with cheese and cucumber/tomato. You can also try giving some boiled sweetcorn with some salt and butter mixed in, it may sound weird but its something I tried with my kids as a snack and they loved it.