Prove that the suffering in Romeo and Juliet is the responsibitlity of the parents.
I am looking for irresponsible acts they committed, actions they failed to take and the result of these behaviors (suffering).
It is interesting that you want to isolate Romeo and Juliet's parents, since there are two other grown-ups in the play that have much more actual effect on the choices that Romeo and Juliet make -- The Friar and the Nurse.
Lord and Lady Montague show their lack of involvement in the opening scene of the play, when they confess to Benvolio that they have no idea why their son is so melancholy and ask him to find out; and Lord and Lady Capulet have no idea what is going on right under their noses throughout the course of the play between their daughter and Romeo.
So, it is hard to hold Lords and Ladies Montague and Capulet accountable for the suffering of Romeo and Juliet, since this is a Tragedy and, as such, the suffering stems from the choices made by the two main characters.
Romeo and Juliet choose to keep their love secret. They choose to marry. Juliet chooses to take the poison and pretend to be dead. Romeo chooses to fly to Juliet's side when he learns that she is "dead" and kill himself. And Juliet chooses to kill herself when she awakes and find Romeo dead beside her. All of these choices are made because both Romeo and Juliet suffer from the same tragic flaw -- They are over-hasty in their actions.
All the above being said, there are definitely things that the parents do (and don't do) which contribute to the reasons that Romeo and Juliet choose as they do. The most glaring is that both families do nothing to end the feud. Much of the choosing of secrecy can be attributed to the ongoing feud, which the parents refuse to end until it is too late, both Romeo and Juliet are dead.
Lord Capulet is also guilty of motivating Juliet's choices when he insists that she marry Paris, when, originally he had protested both that she was too young to marry, and that he would defer to her in whether she was willing to wed. His actions in Act IV really push the haste with which Juliet decides to follow the Friar's desperate plan and contribute to the final suffering by motivating Juliet's action.
But, even though the parents are responsible for prolonging the feud and Lord Capulet pushes Juliet by decreeing that she wed or "starve in the streets," the suffering of the play is still the responsibility of the two main characters and the result of the choices they make out of haste.
For more on the the roles of choice and haste in this Tragedy, please follow the links below.