Hi, if it is genuinely thrush, then sudocreme won't help, , but from experience with my own DD I think its very unlikely to be thrush, if I remember rightly that is pretty unusual for a toddler.
DD had similar problems, & with her it turned out to be different things on different occasions,
1 possibility is thread worms, in that they crawl out to lay eggs at night, & the glue in the eggs is very irritating to some children, they can crawl to the vagina, & can some poor girls to be extremely sore & irritated :( my DD would squeal it was so bad :(
so you need to find out if she's been anywhere where she could of picked up threadworms, can be common in nurseries, play groups etc :(
also look out for itching, especially just after going to bed at night, they come out like clock work at the same time each night, & though tiny white "threads", if you look carefully you will see them......you will need to treat her & the whole family for them & wash & clean everything, towels bedding (above 60*) & door handles etc, if possible steam clean where you can to kill eggs.....google for more info
Otherwise, its possible something irritating her, such as bubble bath, soap, shampoo etc, or possibly something she is ingesting, my own DD first showed with this symptom when tiny, & I was told nappy rash, then thrush due to it not clearing, then excema, none turned out to be right, & we later realised she was intolerant to chemical food additives, "Azo dyes & Benzoate preservatives" been the main ones, & the dyes also show up in bubble bath etc, if its this, then learning about & avoiding certain food colourants will solve the problem :)
Which ever it is, we found, epsom salts & lavender aromatherapy oil & sometimes tea tree oils in a sit bath soothed & helped it to heal, followed by metanium cream, which after trying loads & having it recommended by a nurse friend, is by FAR the best soothing barrier cream, but you may need to shop around old independent chemists, as Boots etc, didn't stock it
Hope you find an answer & some relief for her soon. poor mite & you too, I remember too well how sore my DD used to get, so feel for you both :(