Movies and TV shows starting children are not unheard of as commercial endeavors. I'm not sure why with YouTube as a viable monetizable video platform, parent-driven productions on that platform are fundamentally any worse of an idea.
There might be a time in which we reject such content just as we did with prior widely-accepted behavior that was immoral (like homophobia and racism). Future generations might look down on us because of child celebrities (among other things), and I sincerely hope they do.
You’ve presented no argument as to why it's not alright. Now, this may be a base moral axiom for you, such that no argument is needed, but that's not particularly convincing to anyone who doesn't already take it as a moral axiom.
> There might be a time in which we reject such content just as we did with prior widely-accepted behavior that was immoral (like homophobia and racism).
There might be a time when we see anything in any way you can imagine. That's not an argument that we should.
There might be a time
You need to be over 13 (or above, depending on the country) to create an account. There's no limit on the content itself.
In my opinion, there is no justifiable use case in which videos of children under 13 should be made publicly-available, but YouTube itself disagrees. Case in point: Ryan ToysReview has 18 million subscribers. YouTube thinks it's okay because the videos are uploaded and edited by his parents, while I think it's absurd that anyone makes an income based on sharing videos of their children publicly.