I tend to believe that Friar Lawrence and the Nurse are mostly to blame (while all participants share some of the responsibility) because they were the adults in the situation.
Friar Lawrence could have, at any time, told the children no, that he would not marry them. He could have informed the parents of what was happening rather than offer Juliet the "poison," and he should have remained at the tomb rather than running after Juliet awoke and found Romeo dead. His acts of cowardice and his choice to cater to children rather than behave as a responsible adult is what ultimately led to the deaths of the young lovers.
The Nurse is equally responsible, and for many of the same reasons. Rather than talking sense into Juliet, a child, she encouraged the nuptials and even assisted in secretly making it happen. She was privy to their secret wedding, spoke to Romeo in order to set it up, and suggested Juliet LIE to her parents about where she would be going (to confession, rather than to her own wedding). The Nurse also assisted in ensuring Romeo and Juliet had their night together, in order to consummate the marriage. Finally, when things weren't working out just as the Nurse had hoped, she turned on Juliet (said to just pretend the marriage never happened and go ahead and marry Paris) rather than supporting her and encouraging her to go with the truth to the Capulets.
The Nurse and the Friar did not behave as responsible adults, and encouraged and assisted in the secret wedding as well as the events that proceeded it. Had they made wiser decisions on behalf of the young children, the outcome would have likely been different.