I suppose that a lot of modern children find 19th-century prose hard to understand. It does demand a certain degree of literacy that may be missing in the younger age groups.
In a way I was lucky as a child. From a young age I had a voracious appetite for reading and so read lots of adult books without anyone telling me to. I must have been 9 or 10 when I read
Ivanhoe, for example, as well as several novels by Dickens. I don't think I understood everything, but at that age I had no historical sense and tried to read everything as contemporary writing. It did at least get me used to the language. I even had
Oliver Twist confiscated by a teacher because I was reading it in a geography lesson. (I went to many schools, and hated every single geography teacher I ever had.
)
I thinking reading for oneself early on is one of the best ways of getting to grips with classics. It's really satisfying to be able to pick up any book in modern English (i.e. from the 16th century onwards) and be able get to grips with it.
There is simply no satisfactory general answer to the problem of getting children to like particular set books. A child can hate a book that a couple of years later they might appreciate. That is true of adult reading as well. My taste in reading has changed enormously since I was a young adult. I am now much less scholarly in my reading.