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From your description it seems to be only a hematoma
(collection of blood). Your child requires analgesics for a day or two and some cold
compresses. The swelling may take some time to resolve but the soreness would go in one or two days. Although it does not seem serious but still I recommend you get swellings examined by a Pediatric surgeon and also get a proper prescription for analgesics. He/she might advice an X-ray of the skull, but I don't think there is any need to worry .
Hope that helped
God bless you