at 4ft 7ins a Woman should be between 88 and 116lbs so i'm not sure if this is the same for a child, i'd get her out walking again, playing in parks and soft ball areas to burn off some of the weight. I do think Tae Kwn Do will help her but try and find other things she's interested in doing and more fruit and veg in her diet.
Have you taken your daughter to see the doctor to get things checked out?
I did some research on child weight and they all very this article seemed about average, But not all children are average and many factors need to be taken into account, you said she liked to walk but doesent now, as the Rheumatoid Arthritis clered up or is she still getting some pain,also puberty starts much earlier, so i would get her checked out,
Please keep in mind that many factors go into determining if someone is 'healthy' and that this is just one tool. Talk to your Pediatrician if you have concerns about your child's weight.
A 8 year old (female) child who is 115 pounds and is 4 feet and 7 inches tall has a body mass index of 26.8, which is over the 95th percentile, and would indicate that your child is overweight.
Your child's ideal body weight would be at a BMI at the 85th percentile or below, and so would be about 78 pounds.
That doesn't necessarily mean that your child has to lose 37 pounds. As she gets older and taller, she may thin out some and therefore just need to stop gaining weight, gain weight more slowly or lose some of that weight. Talk to your Pediatrician for more help interpreting these results.
If you think there may be something wrong with her, you should get her checked out. Rheumatoid Arthritis is an auto immune disease in which antibodies attack the joints and sometimes, even other organs; and although it may go into remission, I don't think it ever really goes away.
A person who has one autoimmune disease is very likely to get/have others, such as Hashimoto's Thyroiditis, pernicious anemia, multiple sclerosis, Type I diabetes and others.
If she is unable to lose weight even with proper diet and exercise, I'd most certainly have her checked for thyroid issues (hypothyroidism and Hashimoto's thyroiditis, which is the main cause of hypothyroidism).
I also agree with Elaine -- your daughter may slim down as she grows older and taller, so she may not need to lose a lot of weight, but you should talk with her doctor about it to be sure.