Welcome to the forum familycamper,
You might try adding an inexpensive memory foam (or even egg-crate foam) mattress topper to the existing top bunk. Depending on the width of the top bunk mattress, it's possible that maybe a residential queen-sized mattress topper would be wide enough to be folded (or cut in half & add 2 layers) for extra cushion. Mattress toppers are available at retail stores like WalMart & Target, but sometimes they're less expensive from online stores like Overstock.com.
Hope this helps.
Terri [& DH Kevin]