We've no idea if this behaviour is normal or not. At the moment, there is only the girls account if it. I'm not saying they're wrong. But we have no information on the boys' intentions - and that is the key to whether this is ordinary behaviour (which inadvertently caused unintended distress - the boys are still young/immature) or something deliberate and therefore already worrying. Asking the girls not to tell could arise either from shame (if the former) or manipulation (if the latter)
You'll only find out if you broach the subject. As this family are friends, and as you describe this as a single event, then you can tell the other parents in those terms, and explain the upset to the girls without making assumptions on the intent.
As Goblinchild points out: the boys need to be told again about limits and boundaries. Their reaction might well show what their intent was, giving you further information on whether you need to do anything else (speak again in stronger terms, or sever contact).
Bonsoir makes an interesting point - your DDs are absolutely not responsible for the actions of other children. But if they need to get away from a situation in future, what would they do? At some stage, you might like to consider a self-defence class - not necessarily one to teach them to fight, but one which considers situational awareness and methods to create the time/space to get out.