The best test is for microalbumin in the urine. I tend to use overnight tests.
Children frequently have what is called postural proteinuria.
They excrete more protein when standing and less when sleeping. By using an overnight test, when the urine protein should be lowest, I can believe that an elevated value is abnormal. This is also easiest for children and parents to do. One needs two different samples to confirm the diagnosis of an abnormal amount in the urine. The 24 hour version of the test is, of course, valid too.