What sort of fancy dress we talking about here? If its some elaborate Louise the XV outfit that needs hiring then I agree, 2 days is a bit short. But, if they're simply wanting the kids to have a bit of fun and improvise 2 days is surely fine to knock something up. I cobbled together an outfit in an afternoon for a Madonna theme party not so long ago. I cannot help but think you're being a bit unfair or have an agenda of some sort; you're moaning about minor to non-existent things.
The type of fancy dress is irrelevant, given I'm out of the door before 7am and often not back until after 7pm giving a couple of days notice doesn't really help me. I'm not unusual in working those type of hours and given teachers plan on a half term basis I don't think it's unreasonable to give more notice than a couple of days.