Your daughter's desirable weight should be approximately 10-11 kg. But what is more important and relevant from the way that you describe her is that she is active and alert. The cause for her being underweight is probably the repeated infections. What needs to be done immediately is that you must have a routine examination and a culture and sensitivity done of the urine. Also have an ultrasound of the abdomen done to rule out any problem with the kidneys. You and your doctor will then be in a much better position to give the little one the correct treatment.
She should never be washed with any antiseptic lotion like savlon/dettol. This only makes the rashes worse. Just use plain flowing water and then dry the area thoroughly by patting it with a soft towel. You can then apply Siloderm, Rashfree or any Zinc ointment. Air the area and expose it to the early morning sunlight. Also avoid using any disposable nappies like Huggies etc. Just leave the area open for a while. Feed her less milk and more food. If she is on bottle feeds it would be a good idea to gradually stop these and get her to drink from a glass or mug now. These should always be rinsed with boiling hot water before pouring in the milk. I hope she will soon be better.