Children have a different metabolism compared with adults and this means that they can be very variable in the dose they have to take in order to maintain sufficient levels of the antifungal in their blood for it to be effective.
Voriconazole is a relatively new antifungal so relatively little is known about how it is metabolised in children, though the most recent information suggests that the dose level needs to be relatively high for best effect. In adults voriconazole can be difficult to use as it has nonlinear kinetics (also see talk given to patients by Caroline Moore) which means that a given dose can give very different levels in the blood of different patients in an unpredictable way. Individual patients have to be checked to ensure blood levels are sufficient, sometimes several times. The situation in children is just as difficult, if not more so.
Adding to the problems can be other medication that the patients may be taking and interactions with antifungals are quite common (Antifungal Interactions database).
This recent paper describes two cases of children who were immunocompromised by intensive chemotherapy during treatment for cancer. As a precaution against fungal infection these children were given voriconazole and in line with recommendations it was at a dose higher than that normally recommended. Both children developed severe photosensitivity despite the levels of voriconazole in their blood never reaching that which would cause toxicity in adults. An earlier study of voriconazole side effects in children showed no such problem.
The authors speculate that factors other than voriconazole (photosensitivity is a well known side effect of voriconazole in adults) could also have been important in the development of this side effect e.g. they lived in a sunshine-rich country, they were taking other medications that might have contributed in addition to their young age. However caution is strongly recommended when giving voriconazole in all cases similar to this.