Oh my goodness, I understand your worry completely! I have the same concerns with my grandchildren.
From what I read in the replies, the YouTube Kids app seems like the safest choice - it has built-in filters for different ages. You can even turn off the search feature so little ones can only watch what you approve!
For older children, there’s something called “Restricted Mode” on regular YouTube.
But I must ask - I noticed someone mentioned “monitoring apps” like Spynger. Has anyone here actually used these? I worry about privacy, but I also want to keep the grandkids safe. What do other parents and grandparents think is the right balance?