Here, Juliet uses metaphors and similes to describe her frustration at having to wait for her wedding night. The "mansion of love" stands for her husband, Romeo. When Juliet says she has not "possessed" it, she means that she has not consummated her marriage to Romeo. She means the same thing when she follows with "and though I am sold,/Not yet enjoyed" (ll 27-28). She is anticipating the night, and she is both impatient, excited, and nervous.
In Juliet's last simile, she compares herself to a child who is looking forward to wearing her new clothes (e.g. for something like a festival, a special occasion), but cannot put them on until the festival starts. Juliet says that she feels like that impatient child, waiting all day for the exciting event. Like the child, Juliet too must wait for the event of Romeo's appearance before she can consummate her marriage.
Key point to remember here:
* That very morning, Romeo and Juliet were married (quickly). Then, Romeo ran off and got in the huge street brawl. Meanwhile, Juliet (the virgin) is back at home waiting for her wedding night... thinking that due to her husband's rashness, may not in fact happen now.
"Bought the mansion of a love but not possessed it," = married legally but not carnally.
"sold, not enjoyed." = what have I got out of this marriage yet?
"impatient child that hath new robes and may not wear them..." = like, I've been waiting my entire life for this moment, and now, it is HERE (I have the "clothes" to celebrate) and I won't get to because my husband just murdered my cousin... and all I wanted was a honeymoon. Is that so much to ask?