Unfortunately, psoriasis is thought to be a systemic (and not topical) disease. As much as outside influences can cause a flair-up, even when it disappears it can still be in your system (forever).
You can first try moisturizers, coconut oil, etc., as it could be worthwhile to find a 'natural' remedy. However, in the end it might be a frustrating search.
If it turns out to be a case of serious and systemic psoriasis (cures for which... as far as I know have not been found), in personal experience the best topical solution is a steroid ointment called Halobetasol Propionate. I don't know if they sell it in Argentina, but it might be worth ordering some from elsewhere if need be. That said, psoriasis is tricky and the efficacy of treatments can depend on the person.
I hope it turns out to be the kind that goes away on its own. Importantly, reassure your child constantly because one of the worst side effects can be the unhappiness associated with the aesthetic aspect of the disease.
Saludos