K.V.
My child had high lead levels too. Our house is a 1920s house that we gutted and rebuilt from the studs. The health dept nurse came to our house and all she could guess was that the dust was still in the air from the demolition. Or, paint from the exterior was making it's way into the house.
How old is your child? If she's a baby, when they start to eat "real food" that will help the toxins to leave her body. We washed down the ceiling and walls of my sons room. That seemed to help. We have hardwood floors throughout so we washed all those too. The windows in his room had not been replaced so we painted over them with several coats of primer and paint. We had the air ducts cleaned. The health dept has or knows of places that sell test kits for lead. There are also government programs that will give you money if you qualify to remediate lead in your home.
If her lead level was 10 or higher they have told you she needs to be retested until they are lower than 10. It only took a few months for my son's levels to drop. It's now been several years since then. We yearly have him checked and his levels are >1.