Thank you for your query
A lump in a child most commonly caused by hydronephrosis
that collection of excess urine in the kidney
mostly because there is some obstruction that not allowing the urine to go further (pelvi-ureteric junction obstruction). In 50 % cases it may require a surgical correction but it is a common and a relatively minor surgery with excellent results. So you don't need to worry about it. But i would suggest that you get the ultrasound
earlier. Very rarely it may be tumor
which requires early treatment. That too is curable. You should get in touch with a pediatric surgeon because there may be many other causes of lump too.
Hope that helps
God bless you