I don’t own or use a voice assistant (other than light use of Siri on my phone) but I’d expect that if I’m being signed up for something that will cost money, I’d be warned before having to to confirm that. I’d want something like “I can do this by subscribing you to for per month. Do you want me to continue?” and if I answer yes, I’d want “Are you sure you want a paid subscription?”.
I’m not sure what to do about kids. You could have a password but odds are the kid will hear a parent order something with the password and remember that.
By the way: https://xkcd.com/1807/
