Additional meeting and call-related features and issues within Microsoft Teams for business
you can achieve this by being the organiser going to meeting options, setting yourself as the only presenter - this will auto demote all participants existing and future joins to attendees, as then you have attendees you can select mic off for all attendees - this will literally force everyone to be muted - then you can turn mics back on when you are ready to start and attendees will have option to turn on or unmute themselves.
see screenshot - effectively solves the problem you describe - also solves auto attendees so kids cant mute or kick others.
when ready to start meeting re-slide the allow mic for attendees and they will still be muted just have option to unmute (if you like)