Don't forget too, that the book is specifically written for children and the way children read about this event in the cupboard is quite different to the way us adults, particularly as parents read it.
It is shocking for everyone, but it is probably more horrifying for us as adults to consider. Children are learning new things about the world al the time and take them as a matter if course and matter of fact, usually without dwelling on them. Lots of children will read the words on the page which tell them that the baby is dead and that will seem very sad. They will think at different levels about why that has happened and many will not give it huge thought but quickly move on. It is deliberately not spelled out in this book. As adults, we immediately know what has happened and are horrified, but it is subtly put.
Teachers would choose an appropriate discussion level. Some have mentioned some might exclude that passage (not many I'd imagine) and they would probably talk about how the mother is ill and not able to look after the children. They probably wouldn't strongly push or develop the details of the abuse. Most children will go away knowing the baby has died and being sad and shocked by that, but in the same kind of way they are sad and shocked when they hear about all kinds of other unpleasant things, such as war, hunger, natural disaster. However, again, they move on quickly to think about other things.
This book is a children's classic and continues to be taught because it is suitable and a powerful story. It has shocking elements which are carefully written for children of around 10-13 to bring out a major shocking issue, ut written about in a careful way. It raises issues about parents not always being good parents and it finishes with real hope.
I do agree that in times of remote learning, teachers have less direct contact with children and sometimes need to adjust the order of topics taught and think about how things are covered, but this isn't the same as saying the book is unsuitable or that parents should necessarily be warned about any books being looked at with difficult themes.