Not sure what kids using iPad has to do with them having access to Messenger and iMessage? Or, even YouTube app access.
Speaking as a parent ... It is a parent's job to make sure the software and the content children are using is appropriate for them and is in their interest. If one is relying on for profit companies, then .... I'd rather not say.
As a parent I'm now faced with the situation where I either need to block my kids from 50% of the interactions their friendship groups have and miss out on the media that their friends consume, of find a way that they can do that safely. Which is the crux of my complaint.
If kids are using iMessage I can't say it's OK for my child to communicate with an allowed list of contacts, or it is OK at certain times, or ensure they don't take selfies or send photos they shouldn't.
As it is we have set screen time each weekend, and do our best to educate them on responsible use of messaging but in my opinion apps should have far more fine grained control, especially when targetted at children.
I have so many friends who leave their tablet babysit the baby and feeding them so much multimedia content it's making me physically sick. And it's a lot more widespread, kid sees shiny and soundy and quiets down, then won't eat without a tablet shoved to their face.