Big mistake in use 1 account for 2 kids, how will you split them when they get older, and also raise a issue in who own what and how can they play online together later in life if they want such.

Yes you make more account and yes you hold the Login/PW so they dont screw this up with forgetting them, you can find many steam post with family feaud in who own a steam account, and steam will not fix this for you in who is owner then more then one kids has access to it, until one of them change PW,and not even you can undo it.

see it as friendly warning that might come later and greef with it, when kids fight over account or content, it might not end like this, but it could.

And steam also do not allow shared account, so technicaly you brake steam rules. ( steam is a personal account how can it be personal then own allow other to play at it.