Depends on age. For babies under 2 or 3 months of age, any temperature over 100.4 degrees should be reported to the doctor and checked out. For babies over 3 months, fevers >101, if associated with irritability, change in feeding pattern, change in behavior or other symptoms, or lasting more than 48 hours, should be evaluated.
None. There really is no too high. Fever particularly in children can be 104-106 with many viruses and in many individual children. This will not hurt them, it just makes them uncomfortable. Treat with tylenol (acetaminophen) or Ibuprofen to make them feel better. Remember fever is not a danger it just tells us they are ill.