How to make sure I’m assertive enough in a meeting with subordinates?
You should focus on the outcome and what you need from the meeting.
It can be quite easy to drift from being firm and assertive into being pushy and aggressive. Try to avoid this and focus on what you can do together as a team. And whilst you want to remain friendly, try not to be too chummy.
So it's a fine balance, but it's a skill that can be learned and improves with practice. Don't forget to ask for feedback from your subordinates, as acting on their feedback will help you improve your skills.