Mukuro and Kaito would be conscripted volunteer to be part of the stealth team to smuggle the kid in. Illusions plus diversions is a pretty potent combo. Mukuro also brings some combat skills just in case things go wrong, whilst Kaito is more of a support unit and can throw out some shaky healing songs.

Murtagh would be with Alayne and the army. His dragons would be around roasting things/being nuisances to enemies. Maybe if you're very lucky they can also work magic.

Shougo is likely to be turned and used as a pawn by Sorrow thanks to the blood he ingested and time spent with Akari. I can put up a fight log later for those interested. (He'll later be found in the pub with one hell of a hangover.)