ds was on it a few years ago (think he was 2 and weighed not a lot!) and it made him very floppy! he ended up on about 18mls at night, they kept increasing it as it was doing nothing for the night time spasms but the next day it made the dystonic spasms worse IMO. his trunk was so floppy that he had to work his arms harder which in turn made his dystonic spasms worse so it had the opposite effect on him. i think it is great for kids with spastic cp but i didnt rate it for ds, we had more success with diazepam/nitrazepam at quite high doses with no lasting effects the next day
think most things take a while to kick in, i'd try it for a week at least, you might find she only needs a wee dose.
i always start of with less than they say to begin with now as doc said to give ds 5mls a day of sodium picosulfate but he only needs 1.5-2mls a day, 5mls and the poo was running out of him like water!