Who is the mystery baby?

I was just re-reading the whole set, and I came across something that's puzzled me before.

At the play at the new college on the hill near Plumfield (in chapter 14), there's a baby. The part is played by a baby that all the March women seem to know, and who is quite happy when Meg soothes and coddles him, but there's no mention of a baby before or after this chapter. None of the March women have a new child, none of the next generation have had time to have a baby, so who is this baby?

(Note - the baby is a boy, old enough to sit up, doesn't seem to have words at all, so probably under 1 year old.)