This is a condition called fly-strike.  It is crucial that you act quickly and remove as many maggots as you can.  I know this can be very difficult but from what I was told they maggots can invade the body and kill the chicken if you do not remove them.  Try soaking her bottom in warm water to draw them to the top so you can grab them out with tweezers.
From what I was told you do not want to use too much peroxide because it can darn the tissue and thus prevent healing.
Here is how I treated my chicken.  I gave her the antibiotic Cephalexin 250 mg/2 times a day for 2 weeks and did the following routine; morning- rinse throughly with hose, flush wound with salt water, flush with betadine tea, apply triple antibiotic ointment and evening- soak in warm salt water bath for 15-20 minutes, flush wound with salt water, flush with betadine tea and apply triple antibiotic ointment.  I got the antibiotic from firststatevetsupply.com.  The owner is named Peter Brown and he is none as the "chicken doctor".
Oh and you must remove the chicken from the rest of the flock.  I keep mine in a dog crate on my patio for about 3 weeks until her wound was fully healed.