How should infant hydronephrosis that was in one kidney and now both with much debris be treated?

See peds urologist. Hydronephrosis can be due to obstruction, vesicoureteral reflux, or non-obstructing and non-refluxing. If bilateral, your child is at increased risk. If debris is present in the collecting system/s, then there are certainly concerns. If your child is a male, then there are potential concerns for bladder outlet obstruction as well. See a pediatric urologic specialist soon!
Need cause. Hydronephrosis just doesn't happen - there is usually a cause that can identified. Is there urine reflux, are there posterior urethral valves, is there an intrinsic problem with the kidneys. Once the cause is identified, then you can focus on treatment. Need workup by pediatric urologist and/or pediatric nephrologist before kidney damage occurs.