Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
All Roku devices on a single account will have the same apps available. It's possible to accomplish what you want, but it becomes difficult if you subscribed through Roku for content you want to make available to them. I'll talk about that in a minute.
Here's how I dealt with a similar situation. Read through all this before doing anything.
I created a separate Roku account for kids. If you don't have a second email address you can use for that, you can get a free one from any of a shipload of places: Gmail, Outlook, etc.
Once you create the account, remove the kids' Roku from your current account: https://my.roku.com and log in with Adult account. Scroll to the bottom and Unlink the device.
You can use the same login for Netflix or whatever accounts you wanted, provided you subscribed separately. For example, if you subscribed to Netflix from the Netflix Website, you can use those credentials. If, however, you subscribed to Netflix through Roku, and are billed through Roku (check your Roku subscriptions here) then it's problematic, and probably won't work.
DBDukes https://www.dbdukes.com/Roku Ultra (4660) Roku Streambar (9102) Roku Streaming Stick+ (3810)