Maybe try to show them videos you deem good and/or entertaining yourself. They might even prefer them over the videos they currently like, thus reducing the time spent watching “nefarious” content.
If they suspect that you want to stir them away, perhaps try manipulating the youtube algorithm by playing a lot of “good” or neutral content and engaging with them using their account so that different videos pop up in their daily feed lessening the echo chamber effect.
Ps: This is the opinion of someone who has no experience in parenting