I've been an LPN 14 years & raised a couple girlies of my own. Sounds like she's got a viral tummy bug maybe. When I worked in family practice, we recommended what we called the BRAT Diet bananas, rice, applesauce, toast. Also, if you can get her to drink gatorade, it will help replenish any electrolytes she's losing w/the diarrhea. As hard as it is to do, avoiding dairy products for about 24 hours helps the GI tract "settle" so-to-speak. If she'll drink Pedialyte, that's terriffic, but if you've ever tasted it....blech! It tastes like lightly-sweetened salt water!
As far as the rash on her booty. We use something called "Calmoseptine"--you have to ask for it at the pharmacy. You put a very thin film of it on her booty, & it creates a protective barrier against the skin to keep the diarrhea stool from irritating the skin further.
I'd say if she's not showing improvement after trying these things, contact your pediatrician to see if they offer any other home treatment advice.
Best wishes to you & your's!
R.-a 44 y.o. occupational therapy student, nurse, mom, and wife of a respiratory therapist/paramedic/law enforcement officer(all the same guy... :0) )
email me if you have any other questions, or if you want to let me know how the little tot's doing. Hope she feels better soon, momma!