Was she knocked unconscious when they bumped heads? If yes, then I would take her to your local A&E.

If no, I would watch her at home for the next few hours. If she becomes unusually drowsy or vomits, again I would head to A&E for monitoring. If she is happy playing, I would stay at home and have a quiet afternoon. There is not much a GP can do for a concussion and in hospital, without any further symptoms, you will sit in the waiting room for a while. They will not CT her based on a head strike with no further symptoms and no LOC. Can you get her to clarify the 'dizzy' feeling a bit better? (Tricky with a little one!)

You know her best, does she seem ok to you?