What will you do when some body hit your child in a public place? what kind of a person does this?

I am really upset and still can not digest what happened on the weekend, We went to tea House in a Park in London with my family I was watching all the time my four years old son who was playing close to us, in a matter of seconds as I took a sip from my cola, got a big shock to see a big built man was kicking my son like a dog away, apparently my four years old son was interested in a camera which he was holding and pushed his 6-7 years old granddaughter to look at it! As I saw that shouted at him angrily and said when he has a problem then he should talk to me and not to kick my 4 years old son, he was very rude and he said he did 'nt know where the parent were what else should he do !!!!, I called the police they came immediately and after speaking to both of us told me that he is over protective to his granddaughter and if I was happy that they live immediately and leave us in peace and not seek further action because there was no physical harm was done to my son I agreed, at that time I was in shock phase but now I think they should at least apologize to us and I am still very upset and can not get over it, and I think he is also a danger to his own grandchild when he acts so crazy and mindless