There is a "combined approach" that should work for you, requiring multiple components:
As alecxs successfully tested, blocking
youtube.com will only affect the web browser, but not the YouTube app (and thus neither the YouTube kids app). You could block that in your router, but then it would only apply when your daughter is home and not using mobile data. Do do that directly on-device, you can use e.g. Netguard (yes, the one from F-Droid – the one from Play Store doesn't have that feature as the Play Store policies do not permit, see here).
2. Use a child-protection app
While 1. would work, it would not prevent your daughter from simply "quitting Netguard". So you need something that restricts the apps she can use. For this there are plenty of child protection apps¹ – but you should choose wisely: many of those come with a bunch of trackers (Analytics, Ads etc), which is an absolute No-Go in this context. JoLo seems to be a good choice – it got very good reference from MobilSicher.DE, which never just scratches the surface (aka "looks great and has many features") but also checks what apps do "behind your back" (aka tracking, calling home, extracting data…). With such an app, you can establish a "Kiosk": only the parent can change settings – and the child only gets access to what the parent configured.
So combining 1+2,
youtube.com would not be reachable from the browser, the kids app would still work, and the official YT app could be hidden. On top you get an ad blocker (less traffic, and also protecting your child against "unwanted content"), which you'll certainly welcome.
3. Put the YouTube app into "restricted mode"
You could try putting the YT app into restricted mode and see if that works for you, so the girl can "follow her channels" from there. Not sure how well that works, or how you can keep your daughter from changing that setting (probably only by using Google Family Link which from a privacy perspective can not be recommended, as all data will be logged with Google (again, found out by MobilSicher.DE).
¹ disclosure: link goes to my own Android site