Ahhh…kids. They always fight about who gets to sleep on the top bunk. As a child, I’ve also experienced that. My siblings and I would always try and outdo and outwit each other when it comes to who gets to sleep on the upper deck.
But the lower bunk need not be ignored. Some parents try and make the lower bed more appealing by making it a bigger area and adding all sorts of bedding accessories, including toys, there. You could also put a softer and more comfortable mattress on the lower bunk, to encourage kids to choose that area over the top bunk. It might put an end to all the arguments.