"
It's been so long
Since I last have seen my son
Lost to this monster
To the man behind the slaughter
Since you've been gone
I've been singing this stupid song
So I could ponder
The sanity of your mother
The mother is saying how long it's been sense she last saw her son (Which would be around the 1970's or 80's, before the bite of '87).
She heard of the disappears of the 5 children and believes her son was part of this, so she is saying that she KNOWS someone killed her son.
The mother has been singing the 'Freddy Fazbear Song,' in memory of her son, while she wonders if she's still sane or not.