However, some studies show that, depending on your definition of sentience, a baby might not fit into the category. There are many things (from memory: seeing moral consequences; some kinds of what if processes; understanding analogies) that aren't available to a person until 5-6 years old. Even trivial concepts such as the idea that an object still exists even when it's out of sight are not in the toolbox of a baby.
The original assertion is that people are being logically inconsistent by wanting to sacrifice the young tree, yet reluctant to sacrifice the young child. Based on the premise of tree non-sentience, it is entirely logically consistent that the two cases be decided differently.
The problem with tree sentience in general is that we have yet to define "think" in the context you are using it.
We keep the tree around because of its aesthetic or productive value to us - i.e. the old maple tree is preferable (to us) over the younger. We are inclined, in this case, to preserve the "work" we've already done to raise the tree up to this point. We do not perceive the tree as conscious nor sentient.
The human, however, is, at least in our books. We care about the fact that the child has a lifetime of experiences that it has yet to experience - and we value said experiences, and most of us I think would agree that every human is reasonably entitled to experiencing the highs and lows of life.
I'm not sure if it's valid to harp on people for having different logic and/or morals when it comes to dealing with sentient subjects as opposed to not. It seems to me entirely logically consistent to treat the two differently.