L.M.
This could so many different things.
The first is that the child is hungry and is not being fed enough or often enough. Many kids need to eat 6 times a day (breakfast, snack, lunch, snack, dinner, snack). It's the healthiest way to eat.
Other thoughts...
She is doing this for attention. She can't be getting much attention with only one parent and 2 siblings under the age of 2.
She could have some type of mental disorder or eating disorder.
Why is the fridge locked? Just to prevent her from stealing?
I would contact the child's doctor, as this sounds like it could be something serious that needs to be treated sooner than later.