Hi Dhawal
If your "Parent Account" & "Child Account" are two entities, In Contact form, two lookup fields are related to Parent Account and Child Account entities separately. Right?
You can create a new lookup field ’child related parent’ on contact form, then create a workflow to fill the field according child Account field automatically. if child account field has value and it also has parent Account, you can fill it with value of parent account field value of child account.
However, it will not show parent and child records in one sub-grid, you need create another new sub-grid on contact form.
Here are steps.
1.On contact form, create a lookup field ‘child related parent’ that related to Parent Account,

2.On Parent Account form, create a new sub-grid to related to this lookup field.

3.create a workflow on Contact entity.
Check ‘As an on-demand process’ to update previous records.

2.Expand Add Steps to add actions.
(1) select check Condition.


(2) select Update Record.


Click Set Properties to set field value.

4.Save and Active the workflow.
5.Test.
(1)create a contact and select one child Account, save and wait several minutes to refresh form.
If ‘Test Child A’ has Parent Account, child related parent field will be filled automatically.

(2) on the Test Parent A form, you will see:

Regards,
Leah Ju