I have done research to find that Kaytee's fiesta party mix for small animal food is somewhat harmful. First of all, chinchillas should stick to a very plain diet. However, the Kaytee fiesta mix contains lots of fattening unnecessary foods. Kaytee fiesta mix also is very low in fiber and can cause digestive issues for any small animal. Yes, animals tend to like the fiesta party mix a lot more than normal pet food, but it is NOT suggested that you try this food for them. If they are not eating, there are plenty of other alternatives.
First, figure out what is causing your animal to not eat. Usually, it is due to stress or trauma, and sometimes fear. If you're animal is around loud noises or other animals, you might want to move them into a quieter space. If this doesn't work, try moving the food bowl into another place in the cage so they notice it. If they still aren't eating, you might need to take them to the vet to check their health.
**NOTE chinchillas usually eat a little bit here and there rather than everything all at once. If your chinchilla doesn't eat right away, don't rush into any assumtions. Just give it time and let your chinchilla have space.
I hope this helped