What quotes could I use to support the idea that the adult characters in the play should have known better and guided Romeo and Juliet more appropriately.
The two adult characters who act the most irresponsibly in the play Romeo and Juliet are the Nurse and Friar Laurence. So, locating their dialogues with others may be the quickest key to answering the question. For instance, Friar Laurence acts irresponsibly in his taking upon himself actions beyond his realm; in fact, he breaks his priestly vows by marrying Romeo and Juliet without any bans having been posted as is required by the Roman Catholic Church. So, his words to Romeo,
Come, come with me, and we will make short work;
For, by your leaves, you shall not stay alone
Till Holy Church incorporate two in one. (2.6.35-38)
are completely irresponsible as is his running away from the Capulet tomb and abandoning Juliet when he hears the guards: "I can no longer stay."
Regarding the Nurse, her most irresponsible words are those encouraging Juliet to marry Paris when she knows that Romeo is already her husband.
I think it best you married with the County.
O, he's a lovely gentleman!
Romeo's a dishclout to him. An eagle, madam,
Hath not so green, so quick, so fair an eye
As Paris hath. Beshrew my very heart,
I think you are happy in this second match,
For it excels your first; or if it did not,
Your first is dead—or 'twere as good he were
As living here and you no use of him. (3.5.227-235)
It's going to be a challenge to find many quotes articulating that the adults should have known better and played a more formative role in the emotional happiness of the main characters. Part of Shakespeare's motivation in providing the ending he does is to show that both the young people had a better sense of moral right and wrong and of civic justice than the elders. It is for this reason that they die. Their death represents a sacrifice made or a down payment rendered on the hope for the future. It becomes up to the elders to continue that payment or make good on that sacrifice, something that was not known to them in the course of the drama. Few, if any, speak to this.
I would focus on the Prince's speech at the end. While he is fairly ineffective in stopping the bloodshed, the Prince is able to serve as the source of mediation between both families when confronted with the deaths of Romeo and Juliet. Consider his words:
Where be these enemies? Capulet, Montage,
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love!
And I, for winking at you, discords too,(305)
Have lost a brace of kinsmen. All are punish'd.
The question the Prince poses to both men is profound. In being forced to "see" the "scourge" that resulted from their "hate," the Prince pointedly points to the two corpses as indicating that both men should have known better and led their households in a less destructive manner. The hope that arises from this speech is also an admonition of sorts that the adults should have acted as adults, and not forced the children to act in such a capacity.
1.Virtue itself turns vice being misapplied,
And vice sometimes by action dignified.
2. In one respect I'll thy assistant be;
For this alliance may so happy prove,
To turn your households' rancour to pure love.
3. Go thy ways wench, serve God.
4. Go before, Nurse. Commend me to thy lady,
And bid her hasten all the house to bed,
Which heavy sorrow makes them apt unto.
Romeo is coming.
5. I think she will be ruled In all respects by me; nay more, I doubt it not.
6.Speak not, reply not, do not answer me.
7. Do as thou wilt, for I have done with thee.
8. For it exels your first; or if it did not, Your first is dea, or 'twere as good he were.
9. Ancient damnation! O most wicked fiend! ... I'll to the friar to know his remedy/If all else fail, myself have power to die.
These are just some. :) Hope it helps! God bless. :)))