Two thoughts
1. Have in the advanced properties tab for the payment record, which in this case is opportunities, it is always good practice to check the checkbox �Save parent record automatically when a child record is created�. That way at least when the user goes to add child it will automatically save the carriage.
2. But the bigger tip is to simply include an embedded report on the opportunities record of the children and the user can edit (create) those records directly while creating the parent record before having saved the Parent record. Be sure to make the embedded report editable in form properties. That means that when you edit the parent the embedded child report table a table goes into Grid edit mode.
