As you have to sign on to gain access to iPlayer now, can’t you just sign off so your kids can’t get access?
You can do this, but bear in mind all you need to create a new account is an email address. The sign in is designed to tailor content to the user & help prevent licence fee fraud more than for parental control.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.