I would rejoice at her desire to have my child sealed with the indelible character of the justified and filled with the sanctifying grace and washed forth from the stain of original sin. Now, would I be upset at the arbitrary action that took place behind my back?
Of course! 
But I would perfectly understand her concern, because I know what the Sacrament of Baptism is - perhaps and probably even better than my Lutheran mother-in-law (in the example).
My whole point was to make a clear difference between a believer like you pointing out out of wisdom and prudence that it’s not a good idea to do this (not because of their personal opinion or feelings, but because that is what the Church teaches), and someone who doesn’t care about baptism or religion or God suggesting that it’s not a good idea because it would not feel nice or other such reasonings.
In Italy there is a (quite small) atheist movement that goes around exhorting people to not baptize infants and to send letters to have their baptismal record struck down under the excuse that they did not chose to be baptized. This harms infants of those lapsed Catholics who would have otherwise out of prudence still baptized their little ones, and it harms the very lapsed Catholics who by effectively asking this in writing are risking to be subject to a formal excommunication of apostasy. This is why I was a bit disturbed by the comments: I have heard similar remarks before, in militant contexts, and I am aware of the issues they carry on. We do not know why the parents of the child are not baptizing him if they are Catholic, but it is presumably for a related issue - obviously they do not know how important Baptism is for their little one, or they reject it due to some of those “common sense” ideas about forcing the child into a religion, et cetera.
I do apologize if I sound like I am stirring things up, not my intention at all. I do not approve of doing things secretly either, for it is not the way in which Christians act ordinarily. I just felt compelled to make a statement with no offense intended towards anybody, which is added to the very extensive list of things I will have to be held accountable for one day