Who says anyone needs to inform Valve of your passing? Make sure your children have access to the email backing the account and they're good to go.
Yeah but you need to put that in a package that unlocks in case of your demise, timely or not. Password database and two factor authentication (back-up codes at the very least). Possibly a secondary password database if your primary one contains stuff you don't want those that stay behind to know about.
If this continues long enough there will be plenty of 140-year-olds rocket-jumping through levels in TF...
There are many signals that will inform them you are dead and AI will easily detect them: the change to your gaming routine and purchasing habits and platform usage as they grind to a sudden and complete halt except for the occasional sign-in to play one of your games, and the "age of account" will inevitably betray you.
What happens if Valve don't exist?
What happens when faceID or biometrics is required? Quickly gets very creepy if your kids want to keep playing your games