My daughter was referred for recurrent UTI's (she only had two, but they refer after 2), she was a tad younger than yours but not much.
She had the ultrasound and the dye thing for kidney function (can't remember what its called.
Both are fine and nothing to worry about (the waiting around in the waiting room is the most painful part!).
Not sure if your daughter is just having standard ultrasound or the the one with the dye too.
The one with the dye, they turn up at hospital, inject the dye and then child has to go away for a few hours for it to go through their body to their kidneys then they come back and go into a tube that looks like an MRI scanner. In our hospital they let you hold childs head and put a Dvd on for them to keep them preoccupied., it took about 30 minutes.
The standard ultrasound you need to turn up with full bladder (bit of a nightmare at that stage) and its just like what you might have when pregnant. They then ask the child to wee and then look again to make sure the bladder has emptied fully.
In my daughters case kidney funtions was perfect and the bladder scan showed it was emptying completely so no anatomical problem but because of her age and that she had 2 in 6 months she was prescribed daily low dose antibiotics to prevent another one. Sometime people can just be more susceptible so if scans show nothing then its just one of those things.
My daughter is now 4 and we are still waiting for her next consultant appointment to get her signed off the antibiotics but I actually stopped giving them to her a few months ago because it was giving her thrush and she has not had anything else (plus doc said they would take her off at 4 anyway as risk is much reduced at that age).
Not sure if this is the type of info you wanted but if you want to ask any questions at all about the whole thing feel free to ask anything either on this thread of by PM.