Reset iMac Login Password

As my little 8 year old son grows bolder and smarty, his skills on desktop is freaking me out.

I tried to regulate his gaming time to protect his eye and also not to let him get addicted on games instead more focus on school, however he managed to find ways to get access our home iMac desktop.

Normally, I changed the login PW every time he discovered it. So one day, after i changed the login PW, he found one and he managed to reset it himself using his PW. Now the problem is, he can no longer remember the PW he used to reset. It ended i can no longer reset the password as it requires the previous one to create a new one.

Can someone assist or guide me to show other options or ways to reclaim back my access to reset the password?

I saw some tips on youtube but it's more of resetting the whole system or the storage drive which i doubt possible because i got loads of files stored on it.

Much appreciated for any helpful advice and assistance.

Thank you.

Jay