dd1 has always had a sore vulva on and off since she was tiny. (red raw at times)
I have tried all the following:
we never use anything for washing except E45 junior bath stuff (which looks like bubbles) She sits in the bath and gently swishes some of the bath water into/onto her vulva.
When she was very little we didn't even use the E45 stuff we used oilatum bath oil in the water.
She tended to be a bit slack about getting to the loo and wiping properly, which meant she often had a damp patch on her knickers. I put dry knickers in a pile in the bathroom, and encouraged her to put dry ones on if hers were a bit wet.
Because of her poor hygiene, we did wash daily, but only bath/shower with water or the safe products mentioned. I never use soap in the bath so she isn't sitting in water that has had soap in it, and for the same reason, we only washed hair in the shower, so she doesn't sit in shampoo. But actually at 3 she washed her hair by lying down in the E45 bath bubbles and we never used shampoo.
Nappy cream helps enormously. Good old sudacream, applied generously at night, really would knock it on the head when nothing else did.
We tried the drinks - make sure she has enough, and nice and dilute etc, but to be honest we didn't notice any change.
Both dds still get sore occasionally, but much less now they are older.