Why was Hans going to write in his letter to Liesel and Rosa?

Are you asking about the letters he wrote home?

Pages 436 and 437; Hans stumbles over the corpse of a 12 year old boy named Rudy (not his Rudy, but a young innocent boy who reminds him of Rudy). He wants to write the details of what he experienced that day, the bombings, and the death, but all he could write was, "To my deal Rosa and Liesel."