As an adult with frequent UTI's, and pyelonephritis so bad that I ended up in hospital with sepsis, I honestly feel awful for your little girl. Please push for more tests! I feel so dreadfully ill when I have UTI's and before I was catheterised it was horrendously painful urinating!
My little girl had frequent UTI's after she started pre-school. She was referred to paediatrics and had several scans of her kidneys. Luckily it was caused by my little girl not wiping properly (my little girl has coeliac and unfortunately can have messy poos if she accidentally ingests gluten). We took over wiping when she had poos and was at home and wiped her as soon as she got home from pre-school. This stopped her getting them. I spent ages teaching her how to clean herself properly, (front to back) after the toilet, she is eight years old now and it wasn't that long ago that she stopped needing help. We bought her the fragrance-free/ sensitive moist toilet wipes for children and she has been independently wiping since.
I have spent a lot of time researching how to prevent UTI's and during this research, I read that it's possible for the bacteria from the urine to spread and infect others via the toilet. There was a case of two older sisters, one went to stay with her sister to look after her, as she had a severe kidney infection. Whilst there, she contracted a UTI which went into her kidneys, unfortunately, this turned into septicemia, they were unable to find an antibiotic the infection was resistant to in time and unfortunately, she died. There was an investigation and they found the infection responsible was present in the sister's toilet bowl, which they concluded was how the sister contracted the urine infection. Having read that, and surmised that this may contribute to re-infection, I started to use Dettol all in one disinfectant aerosol spray regularly on the toilet, before and after each time I use the loo. I also use sensitive Dettol soap and wash down below with that, I use disposable dry wipes instead of a flannel or louffa to limit the chance of re-infection. This seems to be helping, unfortunately, I am still prone to them due to long-term catheterisation but I have had long periods without infection which considering I was constantly fighting uti's for over 8 months straight last year is amazing!.
I would also make sure that your daughter's underwear and towels are boil washed to kill the bacteria. Your poor little girl, I know how embarrassing the horrid sulphur/ rotten egg smelling urine is too. Have you had any support from ERIC the children's continence charity?
www.eric.org.uk/
They were very helpful when I contacted them with support for my autistic ds.
I hope you dont mind me linking to some of the things I have mentioned I use to prevent re-infection.
firstly the soap I wash with below
www.amazon.co.uk/Dettol-Soap-Sensitive-100g-Pack/dp/B00BNU08WQ/ref=sr_1_1_a_it?keywords=dettol+sensitive+soap&tag=mumsnetforum-21&ie=UTF8&qid=1517271244&sr=8-1
the spray that I use on the loo, before and after using.
www.amazon.co.uk/Dettol-Disinfectant-Spray-400-Waterfall/dp/B0080N3IRQ/ref=sr_1_1_a_it?keywords=dettol+disinfectant+spray&tag=mumsnetforum-21&ie=UTF8&qid=1517271305&sr=8-1
wipes to use instead of flannel/ louffa when washing
www.amazon.co.uk/Conti-SoSoft-Heavyweight-Patient-Cleansing/dp/B00J3UMQXC/ref=sr_1_3_s_it?s=drugstore&keywords=dry+wipes+personal+care&tag=mumsnetforum-21&ie=UTF8&qid=1517271385&sr=1-3
moist toilet tissue for cleaning themselves properly
www.amazon.co.uk/Pampers-Kandoo-Flushable-Toddler-Sensitive/dp/B00TOZ44CM/ref=pd_cp_75_1?psc=1&_encoding=UTF8&tag=mumsnetforum-21&refRID=C3QA99BF7NAGXKYYG1FT
www.amazon.co.uk/Sensitive-Moist-Softlets-Flushable-Wipes/dp/B071V4V8LZ/ref=sr_1_5_a_it?keywords=sensitive+moist+toilet+tissue&tag=mumsnetforum-21&ie=UTF8&qid=1517271698&sr=8-5
we get ours at home bargains, which is cheaper.
I also advise drinking as many fluids as possible, just water mostly, as juices etc can contribute. Another tip was to get your dd to lean forward after peeing to make sure she is completely empty. Good luck, my heart breaks thinking of your little girl going through what I go through! She sounds incredibly brave!