I think you should tell the Police if you have been advised to. It's not too late. If it had been an emergency like a stabbing, yes there has been a bit of delay but the Police would probably accept the fact that you didn't find out until a day later. They might be building up a log of what these two boys do.
There is a family whose dc attend the same school as ds, one in his year (yr 6), one a couple of years younger. The family is difficult and it has previously been privately suggested that should incidents occur out of school, it may well be helpful to inform the police (yes by someone who is in the know professionally, no not something they maybe should have said, and at no time were names mentioned)
Anyway, that argument aside, yesterday ds went to the local park and came straight back. I assumed there was nobody there to play with and he didn't say or do anything to change that. However he then later on opened up to my mum that as soon as he got there the 2 dc came over and the elder told the younger to smack my sons face - which the younger promptly did.
I didnt find this out until late last night and wasnt able to see my son until after school today to talk aboutit to him. Given the advice we were given (and has been reiterated since, when I informed school today of the incident - I don't expect them to punish for probems outside of school, but my son is slightly more vulnerable and I have a close contact with the school for this reason), is it now to late to speak to the police or not. I am not expecting any immediate action, but it may help overall.