Most major providers (Google, Facebook etc.) restrict children under 13.
Google allows you to create a family – they get their own account but you can manage their apps, monitor screen time and remotely lock a device based on a schedule. Check out Family Link.
The problem with Family Link is that you’re required to have a rather recent Android or iOS device… so it means that you might not be able to give them your old device to use. If you don’t have a dedicated device for your kids, then something else might suit better.