There are some nice fresh materials published by Chess in Schools and Communities: https://www.chessinschools.co.uk/chess-at-home
Chess Tips
| MIDDLEGAME TIPS: |
Don't play a quick move because you can't really decide what is going on, or because you suddenly see a good-looking idea. Take the time to find a good move. I think 15-20 minutes to decide if a Pawn grab is safe is not unreasonable. |