When would you worry that a 1 1/2 year old's fever seizure is a more serious problem?

Depends. Simple febrile seizures are normally are not concerning. If a febrile seizure is prolonged, more than 5 minutes, then that maybe more concerning. Also, febrile seizures are normally generalized, if there is a partial seizure with a fever, then it is not a simple febrile seizure.