There’s no way to set time limits on how long your child watches a specific channel or uses your Roku device every day with Roku’s built-in parental controls. There’s an option to set a sleep timer on a Roku TV, which turns the device off after a certain amount of time, but there’s nothing that stops your child from simply waking the device up again.
If you want to limit your kid’s time on the Roku device, I recommend Bark Home. It gives you the ability to pause the internet on your Roku device when you feel like your child should take a break or be going to bed.