Q: The text of Option A for blessing baptismal water in The Order of the Baptism of Children seems to be the same text as that of the Blessing of Baptismal Water during the Easter Vigil when one or more people are to be baptized during the Vigil. Is there any reason why the music provided in the Roman Missal for chanting that blessing could not (or should not) also be used when Option A is employed to bless the water that is to be used in the baptism of one or more children?
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A: You are correct that the words are the same in those two blessings. As you can also see, there is a difference in rubrics near the end: the celebrant may lower the paschal candle into the water at the vigil, but he puts his hand into the water at the baptism of a child.
So, yes, a priest or deacon may chant the blessing from the Roman Missal when using that option at the baptism of a child.
Is there any reason why he should not? The only ones I can think of are practical: If the celebrants lacks musical skill, if the children require attention, if the baptism takes place during Mass when chanting might add to the length of the liturgy, inconveniencing people who were counting on it taking less time, then, maybe not.
But if it adds to beauty and prayer, yes, it could be done.