I think you are a
I'm like a baby,
but look out -I can bite!
The blades of a lawnmower are usually very sharp.
I depend on the milk you give me
and I will be wherever you leave me.
I think the 'milk' here is a metaphor for either oil or perhaps electricity.
Lawnmowers are generally inanimate so won't move from where you leave them.
I love going outside with you
as you push me in my stroller.
This is the most convincing line for me, the act of pushing the stroller is similar to that of mowing the lawn.
Back and forth we walk
over and over -like a cradle.
A common technique for mowing the lawn is to traverse back and forth over it in strips.
All the while loving the smell.
The smell of freshly cut grass.
Oh. Need diaper change.
When the bag is full you need to empty it, rather like a diaper change.