When I visit the Youtube home page from my browser, all the tiles (recommended videos) show an vertical "..." when I hover over them. Clicking that symbol gives me to the choice of "not interested" and "don't recommend channel." The former supposedly will cause fewer videos like that to be shown (perhaps the topic, not the channel itself). The latter should cause the channel to stop being shown to you.
I don't think you can do this from the YouTube app on Roku. But, if you're signing into the same Youtube account when you use that app, then whatever you do from a browser should apply in the app. You just have to take care of business using a browser, and hope they recommend something from that unwanted channel there. You might actually have to watch some things on that channel to have its videos recommended to you (so you can then do the "... don't recommend channel."
I don't think the Roku's parental controls will give that much control into an app's content. YouTube has a YouTube for kids which I believe would give you more control, easily. But, that might not be the right way to do it. (You might not see all the youtube content. Something might not be deemed kid-friendly, and withheld by youtube to that interface.).
"People are often amazed at how much we’ve done with the number of engineers we’ve got." (Roku CEO Anthony Wood, Austin Statesman, Oct 4, 2019). "Amazed" is one way of putting it.
