As I understand it, 4 children had died before 1911 according to the Census and you have accounted for 3. If there is no sign of Michael in either Census, it would appear that he also died young.
I checked the website for Glasnevin Cemetery, one of the larger cemeteries in Dublin and there is a record for the burial of a Michael Pollard who died at the age of 9 months in 1889. That's all the information given on the free search. You need to purchase credits to get more information but it might be worth a look. The website is www.glasnevintrust.ie