One thing I noticed. CW has spent a great deal of time in all male company (his ship). Now, he is in a predominately female company -- quite a change. If this has already been addressed, my apologies.

Second, he is beginning to see the dynamics between Anne and her friends (so-called). They ignore her or casually use her (Mary). At the first encounter, he had not had time to observe this, hence "does she never dance?" Which could be read to mean "Is she always left alone this way?"

So, when the child jumps on Anne's back, he is doing what they all do. He sees how they treat her and treats her the same. Later, Anne is left by herself for a horrendous stretch of time on the great walk. It's clear that if not for Mary, Anne would not be on the expedition -- she means so little.