Because either 1) it had evidence on it and killer was hoping the water would remove it or 2) killer had no use for the blanket and no reason to keep it. "Safer" to just throw it in the trash bag with the child than dispose of it some other way.
I do agree that the murderer will end up being a female caregiver, but I don't think any of this was done lovingly or with care or accidentally. Someone wanted this child dead. Anyone who loves or cares for a child would contact authorities in the case of an accident, not put her in a trash bag and toss/place her body in/near the ocean in a not-well-known area. If we assume it was her mother, and she checked on a loved daughter and found her dead, some kind of primitive instinct kicks in to deny the child is really gone. She'd likely panic and think maybe EMS can bring her back, maybe she's not really dead, maybe she's in a coma, doctors can help, SOMEBODY HELP MY BABY! Not, oh my kid just randomly died, better throw her away so I don't get in trouble.