Is she old enough to gargle? I've found that, as mundane as it sounds, gargling every hour or two with hot, salty water, even if I just do a couple of gargles each time, gets me through a cold lightning-fast. (I discovered this when I was four months pregnant and couldn't take the usual supplements.)
Of course, be very careful that it's not too hot for a little child's tender mouth and throat!
As big a fan of them as I am, be wary of using herbals, vitamins, etc., for a young child. They can affect children differently. Otherwise I'd advise you to try all kinds of things. You could ask her pediatrician if 250 mg of vitamin C every four hours would be okay. Large doses taken every four hours for a day or so can be very helpful (for adults, it's 1,000 mg every four hours). For young children, though, it's best to give them baths, showers, plenty of non-sugary fluids (no orange juice! It's just sugar and there's not enough C to count), and rest.