Why "Children" rather than "Babies"? Because, for some, "children" may more effectively connote the "personhood" of the young life in the womb and the vitality of that person as well.
Why "Birth" rather than "Life"? Because the unborn child is already alive and awaiting her/his birthday.
So these three little words make three profound statements:
1) The occupant of a pregnant woman's womb is not a "product of conception" or "blob of tissue" or "wad of cells" - she/he is a "person".
2) That person has "the right to life".
3) That "life" already exists and no one has a right to exterminate it.
Read What Abortion Really Is. Also see Right to Life Act.