Translation:De baby slaapt tussen zijn ouders in.
Why does the "in" get splited from "tussen" and goes right to the end of the sentence?
for 'another correct solution' I it had an 'in' at the end. What is the significance of this 'in' at the end of the sentence and without?
It's part of 'tussenin', which means 'in between', 'between the two', or 'in the middle'. So it is a bit more specific than 'tussen'. It's not wrong to drop 'in', though. :)