I get why her mum liked it. This is her daughter. It's going to rip your heart in two seeing any negativity directed at her child. Of course! And she's an innocent bystander in it all.
But I still think she will see the bigger picture too and encourage them to smooth it all out. To keep a business that supports the children going. Taking away her like was the right move, although I'm sure she loves comments that defend her daughter. I would. Very hard to deal with it all when your daughter IS the business. Hard to draw the line.
Brand workers are going to be all over all of this. They need to very quickly come up with a very robust stance on how they deal with negative comments. Either a blanket policy that they all get deleted, no matter who it's directed at, or they all stay as long as it's not obvious abusive trolling.
There are some smaller YouTubers I watch, with a fifth if the audience who get good brand deals. These vloggers are squeaky clean, great engagement and very little negativity across forums. That tells me that this aspect of it is important to brands too so they must, must do a major clean up operation