Is Baptism of a child still valid if one of the pouring during baptism misses?
Not finding an adequate source of information to truly answer this question with a confirmed source, I am going to answer this with an example of baptism with a slightly different dubium in the administration of this Sacrament. If a real doubt truly exists in this matter, one can always write Rome in the form of a dubia.
The baptism would be valid in the above example because the intention of the individual was to administer the sacrament with the intention that the Church willed that the baptism be preformed. Thus the form, matter and intention were preserved, even if one of the pourings were to miss the head of the child unintentionally. If the administrator of the sacrament noticed the miss, it would be best to repeat the words and pouring of water than to be plagued by doubts later.
Now let me give you an example with a twist. This really happened, but I can not find a source of information at the moment.
About 30 years ago I was on an eight day retreat officiated by Monsignor Raymond-Marie Tchidimbo while I was in France.
In one of his conferences, he recount on of the story of his own baptism in the village ware he was born. Bishop’s story goes something like this:
Many years after his consecration as a bishop and of having ordained many priests to the priesthood, he decided to visit the village where he was born and baptized. Having done so, three women recounted how they baptized the young Raymond-Marie together. One of the women held the baby. Another one of these ladies poured the water while the third woman pronounced the words to baptism over Raymond-Marie.
When Bishop Tchidimbo heard this news, he was immediately horrified because he presumed the baptism was invalid because the administrator on the sacrament was not the same person saying the words of baptism and pouring the water at the same time.
He immediately refused to celebrate the Mass, for he feared his own baptism was invalid. If so, every priest he ordained would not be a priest at all. What a dilemma?
He thus wrote a dubia to Rome and asked if his baptism was valid or not?
Rome responses in the positive, stating that by the internal forum had been preserved because they had acted, in spite of being outside the norms for the administration of the sacrament of baptism, they had the proper will and intention to baptize him as the Church would desired In this particular situation.
Thus the baptism of Raymond-Marie was valid.
Given the nature of the above circumstances regarding baptism, I do not believe the Church would declare a baptism invalid because someone inadvertently missed the head of an infant in the course of administrating the sacrament of baptism. The intention to do as the Church willed is still preserved.