Alfons was giving his in-laws a chance to breathe without dealing with a hyper toddler - in other words, a typical Elric child. He got up from the living room floor where he'd been working on keeping Joseph's attention on his toys and not the shiny expensive electronics or anything else a toddler shouldn't get into at the sound of the knock.
"On my way!" he called out as he scooped Joseph up and carried him on one arm as he opened the door. "Guten tag, you two." Then he paused. "What is wrong?"