Forcing anyone out of the lifeboat is murder, period. If someone chooses to sacrifice themselves for the benefit of the others, good for them. The baby cannot make the choice for itself.
On reflection, perhaps a better reframing of this thought experiment (to avoid many of the issues that others have brought up) would be that a murderer has broken into your house and tied you up, along with a baby and a highly accomplished but relatively young person, and offers you the choice of which one of the others he's going to kill, along with a credible assurance that he'll kill all of you if you don't choose one (and you're not allowed to pick yourself, just because), and he'll really let the remaining two of you free once you've made a choice. Further, you're not allowed to discuss with the others, so there's no opportunity for anyone to sacrifice themselves.