Medicine at Peoria responded to your questions:
is a very uncommon disease, with about 2-3 cases per million. Thus the chances of your child developing SJD from Motrin is quite low. If your child has taken ibuprofen before without an adverse reaction, the subsequent development of an allergy is less likely but not impossible.
(For example, lots of people develop allergies to penicillin, but after having used it successfully for years prior). As with all medications, the risks of the medicine should be weighed against the benefit expected. Since acetaminophen (Brand name, Tylenol) is available for fever reduction in children and infants and is not known to have this potential adverse reaction, one might choose it as a first line of treatment."