Not sure if you have kids, but the devices didn’t start out as being theirs. It was my iPad, they started using it more as they got older, and eventually I got a new one for me. I should set it up properly, but it happened gradually and I have not gotten around to changing it… and now all the apps and accounts and settings are tied to my Apple ID.
The more important question is how old are yours? Toddlers can't read so much, so it's easier to understand not making them their own accounts it a priority. On the other hand, if they're teenagers, seeing private texts between their parents could be deleterious to their mental health. Still, they're not getting younger. The only way out involves some level of pain, but the sooner they get their own accounts, the less pain there will be.
