I always include infant or child deaths.
They are a part of my families journey, even if I never met such family as they existed long before me.
Although this was modern, my 2nd cousin thanked me for including her son who passed in infancy on my tree.
My 2nd Great grandma had 3 children who died in infancy and childhood, confirmed by birth certificates which my great grandma had kept a secret from my grandfather.
I even have some family photographs which include babies (still living in the photos

) who died around 2/3, and so I have added both their names and their photographs to my tree so that they are not forgotten.
Including infant deaths provides to me a greater understanding of my families journey as well as who were commemorated through these infants names, many babies named after their grandparents.
It may make trees appear more cluttered, but to me, they are still a part of my family. In fact, I am glad in a way when I find an infant death. I am the only person in my family to be interested in family history. So finding a child or infant death I always think, when was the last time somebody thought of them, and so there they are, proudly on my tree!

I have even searched newspapers to give them a proper date of death instead of just a quarter in which the death was registered.