Took a look on Genealogy Bank and found obits for potentially 3 out of the 4 unknown children born to Patrick & Annie that died young.  Most of these obits however named the father Patrick "K."; all the recent info rec'd indicates that Patrick's middle name was Timothy.  The last born child, Leo Timothy Mullin, has his father's middle name.  The 3 children whose obits I found were named James Leo [I don't think Leo was that common and it was also the name of Patrick & Annie's last child]; Andrew P. [Annie's brother was named Andrew, and P could stand for Patrick]; and Joseph P. [middle name Patrick again].  

I studied the city directory entries I'd extracted for Patrick in Baltimore from 1882 through the early 1900s.  I was able to match 2 out of 3 addresses to the childrens' obits.  [The other obit did not specify whether or not the address shown for the funeral was the parents' home address, and it was not an address shown anywhere in the Patrick Mullin listings.]  Most of the city directories listed Patrick with no middle initial.  However the 1895 entry had him listed under both surnames Mullen and Mullin, same address as the 1895 obit for his son; the Mullin listing showed the middle initial "K".  The 1884 listing showed Patrick K; the following year he was listed at the same address, no middle initial.  The same for the 1898 and 1899 listings.  There were various Patrick Mullins listed in the Baltimore directories over the years, some of which were fairly easy to eliminate.  The above were the only listings for Patrick with the middle initial "K", and they matched the probable addresses for "my" Patrick.

Date-wise, the obits indicate birthdates for 3 children which fit in pretty well with the estimated birthdates of the Mullin children listed in the federal censuses.  There are one or two instances where a date does not fit, but the Keys family tree in those cases does show different birthdates for those siblings.  The federal censuses are not always accurate; not sure where the info came from for the Keys database though.

While still researching, I am going to tentatively add the 3 children who died young to my database.