Given: At least some unborn humans are persons.
1.) Persons have rational souls and so are ordered to the beatific vision.
2.) Where there is order to beatitude, something is responsible for attaining this beatitude by its proper means, which in the case of beatitude is baptism or some intentional act with the force of baptism.
3.) For the young, the parent is responsible.
4.) One cannot be responsible for doing something that cannot be done.
5.) Since baptism of the unborn is impossible, the parent must be capable of, and responsible for, some intentional act with the force of baptism.
Just Thomism 