First, I can understand, given the family history, why you are all concerned and can tell you that kidney and bladder cancer
are very rare in children, with Wilm tumor being the most common, and fortunately the more curable solid tumor cancer in children (95%+).
Second, blood in the urine is something we see from time to time in children and a UTI is the most common cause but it is often just an irritation of the urethra (tube leading from the bladder for urination) and just goes away on its own. This is common in kids who sit in a tub with lots of shampoo or bubble bath in it, or after swimming in a heavily chlorinated pool (all public pools).
All I generally do, if the rest of the physical exam is normal, is recheck their urine in a few days and generally the blood and irritation have cleared and no further evaluation is necessary. This seems to be a more common occurrence in little boys than girls: pain with urination without anything on the UA, but I always do a culture to make sure nothing grows over 48-72 hours in the lab.
Again, considering the niece with the tumor, I understand your concern but can tell you first, these are not hereditary cancers, and second, nearly 100% of boys with these symptoms clear spontaneously. Just follow up with his doctor and try not to worry about it as I can tell you that over my many years as a pediatrician, having seen many children with cancer, including at least 6-8 with Wilm Tumor, none of the presented with urinary complaints but rather with abdominal pain
or a mass found by the parents when bathing their child.