And welcome to the site. It's difficult at the best of times to get a diagnosis whether it's a child or an adult. It can be a long slow process.
Hopefully your doctor is aware of this and will not right off the possibility of lupus with out further testing. If they are familier with lupus they will continue to check.
Having said that, please don't take me wrong, I am NOT wishing lupus on your child ... only to let you know it is difficult to gain one.
There is not quick answer and it can take redoing tests to get the diagnosis. The important thing is to get it identified and begin treatment.
In the mean time make sure to limit exposure to the sun. Use a good sunscreen and get wide brimmed hat and good sunglasses.
Exposure to sun light can bring on a flare and make symptoms worse.
Plenty of rest, eat well. The better people with lupus take care of their health and the bodies the better off we are.
Swimming is a good way for us to keep our muscles strong and our joints moving with out causing too much stress. I know it's hard to think about moving sore joints as being a good thing, but it is.
There is alot of good information on this board and very helpful and friendly members.
I'm sure someone will be along who can answer you questions about children and lupus with more detail than I.