I’d coach the first action after conceding.
“Heads up” is well meant, but it gives them nothing concrete to do with the feeling.
After a goal, give them one simple reset: get back into position, find a teammate, and make the first action together. That might be a short pass, a run or simply talking to each other.
In training, you can create short games where a goal is followed immediately by a restart so they repeatedly practise recovering from the emotion.
I would also be careful with the adults. If the coach’s body language changes dramatically after the second goal, the children will copy it.
You are not trying to make conceding feel good. You are teaching that the game continues and the next action is still available to them.
Resilience at U9 is often built through very ordinary resets, not motivational speeches.
