I like Bloggsworth's edit, but I think it would be stronger without the ending stanza. It seems to try too hard to be clever. Here's how an edit would look (also changed the linebreak from "so they left it" to "so they left / it" because it is stronger and has a double meaning):
For my childhood,
I received from my parents
an empty box.
Cut from plain brown card
a rough cross,
which when folded, made
an empty box
fixed with sellotape.
They were busy
demolishing their own lives
with drink and despair
so they left
it at the bottom
of my bed.
I open it every day
of every year
just in case, maybe this once,
it has been filled
with love.
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