How about stressing the benefits and flattering them by saying things like "Oh, you guys are great, you'd be the best roommates ever." Or shameless emotional blackmail, along the lines of "Oh, please room with me, I don't know what I'd do if you say no, I'll just be miserable." I've witnessed people doing this kind of thing, it's not unusual.
Dunno if that helps, just throwing stuff out there. I've never had to do this myself - I can't believe that young adults in America (I take it you're in the USA?) have to share rooms when they're at college. I mean, I've lived in shared flats for years, but sharing a room? That would drive me nuts.
I'm a woman. I don't like children. I don't hate the way I look.