"The child beat the bed."
Translation:Barnet slog sengen.
In the US we say, "hit the sack" as in "going to bed.." same context or actually physically hitting the bed?
Yes, I think it means to hit the bed with an object.
The phrase shows up in a Danish version of the Brothers Grimm story The Valiant Little Tailor: "... as soon as it was midnight, the giant got up, took a great staff of iron and beat the bed through with one stroke ..."
Do an online search for "slog sengen". Click on the google book "Folke-Eventyr" dated 1863 and go to page 88. You can see "slog sengen" in context, in an older typeface.
Note: I did an online search and the phrase is often used with adverbs: "slog sengen ud", "slog sengen ned", slog sengen over".
He might have been throwing a temper tantrum and pounding it with his fists...