Ah, thank you for the clarification.
Note, however, that you are citing a book on etiquette. This would be rather like citing Emily Post’s recommendations for behavior and claiming that her pronouncements were based on Constitutional or statutory Law.
I would not be surprised to discover that some bishop*, somewhere, based on his own personal perceptions of etiquette, directed a person to modify his or her behavior to comport with his view of etiquette in a Catholic context, (although I have no knowledge that such an occurrence ever actually happened), but there is no directive from the church in the way of decrees, bulls, conciliar documents, or canon law that attempts to tell a grieving parent that they must or must not mourn the loss of their child or set limits on that period of mourning.
I don’t think the question in the OP was silly, (and don’t recall anyone else thinking so); I just think that it was a misunderstanding on the part of someone who conveyed the information to the OP.
*(I would almost expect to discover that some officious pastor took it upon himself to demand that one parishoner or another conform to his personal view of etiquette, but he still would not be speaking for the church.)
Monitoring the antics of the Biblical Inerrancy crowd gives some very good reasons not to take The Bible literally…
Total hijack, but having them criticise someone for “getting out of mourning clothes too soon” is a common resource in 19th century and up-to-mid-20th century Spanish novels to caracterize the kind of people we call beatos - not as in “someone who’s been beatified but not sanctified”, but as in “a pharisaic wannabe saint”. I’ve had people walk up to me just to ask, after looking both ways “your mother is not wearing mourning?” - in the year 2000.
I would guess that this thread is not really the place to get into the issue of how Catholicism views or uses the bible.
Regarding stories set in the same time period, it is also true in regards to English language novels, (E.g., Gone With The Wind). However, those are still social conventions, not religious directives.
Mourning clothes are no longer a big deal in the U.S., but there are still busybodies who make a point of noting “inappropriate” (in their eyes) behavior.
Part of the reason that people didn’t have photographs of living children was that exposure times for early forms of photography required very long exposure times - and children couldn’t sit still that long. Photos of dead children could be taken, so the majority of photos taken of children in the Victorian era were of the dead.
Oh yeah, and like any other social convention, they change with location and time and the people who want to Guard and Defend them are a lot more extreme than the actual reality around them. I mean, the last time I saw a non-Roma in full mourning garb was… sometime in the '70s maybe? and these people are still expecting the rest of the world to wear it - but you know, I’m sure those same busybodies of mine have had deaths in the family (one of them’s a widow, another lost a son), yet I did not see them in full mourning garb. Tch tch… :sadly shaking head and looking mournful at the decline in appropriate mores:
As someone who has virtually no understanding of Catholicism, I ask - how does what you say here jibe with the notion of Original Sin? More specifically, are not children who die at a very young age, and who thus may not have had a chance to be baptized, denied entrance into heaven (and thus condemned to be in limbo)? If I am correct, is it not at odds with your conclusion regarding “the child’s presumptive entrance to heaven”?
In short, it doesn’t. The fate of children who die before baptism has never been clearly established, though throughout history various theologians have pondered the question. An overview of the most common theories can be found in The Catholic Encyclopedia. Wikipedia has references to some more recent pronouncements by the Church on this topic, though they pretty much repeat the claim that the status of unbaptized infants is unknown.
Most children born into Catholic families are baptized as infants. It is even permissible for a lay person to baptize a Catholic who is dying. Some other Christian denominations wait until the a child is older and can make an informed choice to be baptized.
Since the discussion is of funerals of children in Catholic families, the presumption is that the chidren would have been baptized as infants, which removes the taint of Original Sin, leaving them sin free until they get old enough to sin on their own. 
As to the whole issue of Original Sin, and what happens to the unbaptized: Limbo was a theological construct for those who could not find a way to reconcile statements that no one could be saved without being baptized with the clearly unjust notion of condemning a child to hell. (This idea also extended to good pagans, by the way. It is the logic behind Dante’s First Circle of Hell, with lots of good and decent ancient Romans and Greeks and Hebrews wandering around in a pleasant neighborhood with no torments beyond their inability to see God.) While speculation produced the concept of Limbo, the church never embraced the idea and it has never made it into Church teachings, (despite what some of our second grade teachers might have claimed).
More recently, (as noted), the church has recognized that God is not constrained by human understanding and that while there is a presumption of a necessity of Baptism, there is also a presumption of both the justice and mercy of God that permits Him to accept anyone He chooses into Heaven.
Also tangentally, in Indonesia they do not name their children for 90 days, nor let their feet ever touch the ground. They are considered to still be among the God world, for that time. Cultural, but surely related to infant mortality.