Albert and David bravely spoke up to the Major and Sergeant. What did they offer to do to help Joey get better?

From the text:

Major Martin looked hard at Sergeant Thunder, taking his meaning, and then nodded slowly. ‘All right, Sergeant. You made your point. Of course there’s a chance,’ he said carefully. ‘But if once we start with a case of tetanus, then it’s a full-time job for one man for a month or more, and even then the horse has hardly more than one chance in a thousand, if that.’

‘Please sir,’ Albert pleaded. ‘Please sir. I’ll do it all, sir, and I’ll fit in my other horses too, sir. Honest I would, sir.’