Should the adults tell children when a goal has actually gone in?

Yes. If the children genuinely do not know whether a goal has been scored, the facilitator can simply say “goal” and help them reset. That is very different from refereeing every tackle or telling players what to do next. The current 3v3 guidance gives the pitch facilitator responsibility for helping players understand the rules and stepping in when needed. A ball going through a pop-up goal so quickly that half the players miss it is exactly the sort of moment where a short bit of adult help keeps the game moving. I would make it as low-key as possible: “Goal, back to the line, off we go.” If somebody on the side calls that refereeing, I’d explain that the adult is there to support the children with the game rules, not to control their football decisions.