Tax management project where 2 objects are tax payer details and tax paid per month then which is parent and which is child . please let me know

Hello Mounika , 

I would have checked both objects and checked the relationship if any exists on both objects . 

Check if there are any master detail or lookup relationship between them . One of the should have above relationship . 

If there is MD relationship between parent / child then in related lists you can confirm the related object is child .