1
The main disadvantage to be an only child is that you don’t have anyone to play with.
The main disadvantage of being an only child is that you don’t have anyone to play with.
Use the/one/main advantage or disadvantage of + noun/-ing: ‘The main disadvantage of living alone is the cost.’
2
There are some advantages and disadvantages to be an only child.
There are some advantages and disadvantages of being an only child.
There are some advantages and disadvantages to being an only child.
We can also use advantages/disadvantages to + noun/-ing, especially after there is/are: ‘There are several advantages to working from home.’
In this pattern, to is a preposition, so it is followed by a noun or the -ing form, not the infinitive: to being, not to be.
After have the advantage/disadvantage, we normally use of + noun/-ing: ‘This method has the advantage of being simple.’