Hello, I understand your concern for the sunburn in your 9 year old son. It is quite often seen that due to acute sun toxicity one might develop welts or rashes on hands other sites of body. In your kind of cases I prefer to suggest cold compression first and foremost. I suggest them to do cold compression for two to three times per day. Application of calamine lotion very thinly over helps in soothing of the affected area. It is most important thing to do stringent sun protection so that further harm and progress could be halted and the acute part of sun sensitivity could be reduced. In case of itching and pain an antihistamine such as cetrizine or loratadine can be taken for two times per day. All the best.