Dogs can eat Jello as long as it is homemade Jello and does not contain Xylitol.
Xylitol contained in store bought Jello is toxic to dogs so they should not eat any foods containing Xylitol which is an artificial sweetener.
If a dog is given Jello and ingests Jello that is store bought they could die.
The reason is, jello typically contains the artificial sweetener xylitol, which is unsafe for your dog.
Xylitol is extremely toxic for your dog and even small amounts of it can cause hypoglycemia, seizures, liver failure, and death.
If you make homemade Jello without the Xylitol then it is safe for dogs.
But store bought Jello should never be given to your dog.
It's not worth the risk to your dogs health and life.
Even though they may want the Jello it should never be given to them.