This may be a special situation.
My soon to be wife nannies for 2 wonderful kids. One is age 3 and the other is a 12-month old. As our flower girl, we have the 3-year old. Her baby sister "the 12-month old" is almost always included in everything the 3-year old does.
Both the bride and bride family are ok with my fiancée's boss's kids both being invited into all wedding events and I'm fine with it as well because that's what my bride wants and will make her happy.
My parents, however, are strongly advising this is an awful mistake to include the 12-month old in the ceremony as the only invited baby and that all this will do is ruin the ceremony with the constant sound of "dadada and gagaga and crying".
What is the common custom on including a baby in the ceremony? It may even be cute if the 12 month old is just able to walk down the aisle