I would ask for more details, ask him how he knew the man was videoing him and why it upset him, and ask him to show me which flat. I would decide what to do when I knew more. I would not go to the police at this stage.
Just makes me feel a bit sick! Why would a stranger want to video children playing out (if that is what he was videoing). There are quite a lot of children who play out on our street and the street in front of us( we live at a T junction). Most of the dc are 8yrs or younger.
a) Because they are possibly a nuisance and he wants to show proof of this.
b) Because they are cute and funny and he loves the fact that he lives in a place where young kids are free to play outside and wants to show his friends and family.
c) Because he's hoping something funny will happen and he can send it in to that video show.
d) He isn't filming the kids, he's filming something else outside.
e) He's got a new phone or camera and is just filming random stuff to figure out how it works.
I would be most worried about a child of this young being upset by something like this. Has someone made him feel paranoid or perverts and peadophiles and made him feel that this is a bad thing? I'd feel sick if something like this upset my child too. Make sure your child knows that if he doesn't feel comfortable about it he should just com inside.
Just like to add that I got a funny look from some people on the beach last week when I was holding up my phone with camera pointing towards them. I had not even thought about them, was using the camera on the other side to take a picture of me and the beach, I have no images of them on my phone and nor do I want any although it must have looked so like I was taking their photo!