Arland closes and opens his eyes one single time and then nods and says "Of course we will. And I will assist you with all I have in my arsenal every step of the way."
Ciel knows by Arland's promise to Catherine, what he's about to do. He didn't necessarily know if Catherine was that injured, she didn't seem it to him at first when it happend, and he thought the doctors fixed her up pretty good once they reached the ward, not to mention Arland's assistance beforehand, but just the same Ciel's aware about what's about to take place. Knowing they are in a public place though, anyone of the doctors, considering the hospital ward is where they are at, could come in at any moment. So even though he doesn't feel up to being without him at his side just yet, Ciel sends Sebastian out to guard the door as Arland uncovers his demon power seal to show to Catherine.
Sebastian sees Arland unconver it, and then sees Ciel's still pale and suffering filled look when he turns at his master's command. But Sebastian does as he's ordered to do and goes to guard the door, turning to Arland though motioning to Arland to watch Ciel as well while he's making his contract with Catherine.
Arland nods to Sebastian, indicating he will keep a close eye on Ciel while Sebastian guards the door, as Ciel is sick and knows he can't leave the room himself, but that Ciel still wants to give Arland himself and Catherine as much privacy from the nurses and doctors as he can while they seal the agreement and sign the contract with eachother.