Just to add my 2 cents...you're not alone!
It's natural to want to give the little babies some treats...if only to get them to grow accustomed to our faces!
I've found that my chicks, so far, have turned their beaks up at fruit--watermelon, blackberries, tomatoes (I was also trying to unload some extra yield before they spoiled!)
Yogurt was a no-go, but they did seem to like kefir (Although I don't know if they preferred to walk in it or eat it!)
What they REALLY love are dried mealworms! They go bonkers for them, and it has helped me to "tame" at least one of the chicks. Just make sure to have sand and/or chick grit available, and it's fine for them to eat a few, as long as their mainstay is still their crumble.
A few crickets have also found their way into the brooder, and made for an exquisite treat as well, with a lively game of keep-away resulting!
I'm thinking of picking up a few live crickets at the pet store, since they are becoming scarce in the garage! LOL Plus, the two biggest chicks keep "winning" the crickets, and I would like them all to have the opportunity to feast on a cricket of their own!
Chicks who are raised by free-ranging hens have early access to many items besides their crumble, and they tend to be quite healthy...