You dont - you make your Roku accounts at Roku.com. You'll have to make a new account with a different email address.
You'll need to remove one of your devices from your current Roku account, which is in effect a factory reset - you'll need to re-activate it using the new account/different email address, and re-add all the apps you want, and re-authenticate them.
At that point you'll have two accounts and one device on each account, and therefore separate app installs for each.
If you want different lineups on the different Roku devices, you need two accounts. That's easy enough.
However, here's where it gets tricky.
Roku lets you subscribe to content through Roku Pay, on the device, and charges the payment method that's tied to the account. Which means you should ensure you have a PIN set up on the accounts. You don't want the kids accidentally subscribing to something.
The other thing is that if you run a multiple Roku account setup, do not subscribe to anything through Roku. Let me repeat that: do not subscribe to anything through Roku.
Roku subscriptions are tied to the Roku account. For example, if you subscribe to Netflix through Roku, you can only use Netlfix on a Roku device tied to your Roku account. That means it wouldn't be available on the kids' Roku.
To solve that, subscribe to the service directly. For Netlfix, simply subscribe at Netflix Website, and use the credentials to log in on Roku (or any device). Don't subscribe on the Roku, but Log In instead. You can use your direct Netflix subscription on any Roku device on any Roku account. And, on non-Roku devices, too.
This is true for all services; Hulu, Disney+, etc.
DBDukes Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." Note: I am not a Roku employee.