My 2 month old child has been diagnosed hydronephrosis. last month one kidney 9,5 mm, other 7 mm. now, 13 and 8 mm. can it be cured by doing nothing?

Its a problem: Hydronephrosis usually means there is something blocking outflow & creating back pressure on the system. This leads to ballooning of the collections system & fluid retention. Exploring/removing the obstruction is often necessary or the kidney function declines over time. The treatment must be determined by those with all the relevant testing.