Discuss the relationship between Jessica and Shylock. Are we meant to sympathize with the moneylender's daughter? Does Shakespeare seem ambivalent in his portrayal of Jessica?

Jessica's decision to elope with Lorenzo makes it pertinently clear that things at home are not happy at all. In Act 2, Scene 3, she exposes her cynicism when she tells Launcelot "Our house is hell" when she comments about him leaving their service. During this conversation, she gives Launcelot a letter for Lorenzo, her Christian lover, and also exposes the fact that she is afraid of Shylock by stating:

I would not have my fatherSee me in talk with thee.

She provides more evidence of her strained relationship with her father when, in a brief soliloquy, she says: 

Alack, what heinous sin is it in meTo be ashamed to be my father's child!But though I am a daughter to his blood,I am not to his manners.

She is evidently embarrassed to be Shylock's daughter, for, as she says, she does not behave as he does or have his attitude. She furthermore mentions that marrying the Christian Lorenzo will end her suffering. Jessica seems to be truly determined to leave her father, and she is desperate enough to relinquish her Jewish faith to marry a Christian. It is also apparent that she does not care what her father thinks. She knows that he hates Christians with a passion but is prepared to face his censure, and even being disowned, for having a relationship with one, let alone marrying him.
In Scene 5 of the same Act, we witness just how controlling and manipulative Shylock is with her. He repeatedly calls on her when he comes home. This is a clear indication that he sees her as one to satisfy his whims, and he comes across as quite impatient and demanding.

What, Jessica!—thou shalt not gormandise,As thou hast done with me:—What, Jessica!—And sleep and snore, and rend apparel out;—Why, Jessica, I say!

When Shylock later says, "Jessica, my girl, look to my house," this term of endearment ("my girl") seems to indicate that he does have affection for her. However, his true nature is exposed later in the play when he discovers that his daughter has disappeared with a large quantity of his most precious jewels and a substantial sum of money. In the scene quoted above, though, he sternly instructs her to lock up his house and to not "gaze on Christian fools with varnish'd faces." He wants her to avoid any contact with Christians and appears to be quite protective and demanding. This fact is later confirmed when he tells her:

Well, Jessica, go in;Perhaps I will return immediately:Do as I bid you; shut doors after you:

When he leaves, Jessica comments:

Farewell; and if my fortune be not crost,I have a father, you a daughter, lost.

Her statement confirms her resolution to be forever rid of her father's presence and influence. She has planned to run away with Lorenzo the same evening and seems to have no regrets about having made such a drastic choice.
In spite of his seemingly affectionate reference to Jessica as "my girl" (mentioned earlier), Shylock's real sentiments about his daughter are exposed in Act 3, Scene 1 when he says the following about her:

I would my daughterwere dead at my foot, and the jewels in her ear!would she were hearsed at my foot, and the ducats inher coffin!

This shockingly cynical and insensitive response demonstrates that Shylock cares more about his material belongings than his own flesh and blood. He surely must also see Jessica as just a possession, albeit one that is less precious than his jewels and money. His amazingly callous attitude is confirmed later in Act 4, Scene 1, when he tells the court:

I have a daughter;Would any of the stock of BarrabasHad been her husband rather than a Christian!

In this statement, Shylock is alluding to the rebel Barabbas, whom Pontius Pilate released instead of Christ. Shylock believes that Jessica has betrayed him and can therefore never be forgiven.
As Jessica herself states, the broken relationship between father and daughter has been brought about by her father's demeanor. It is evident from the text that he does not care about her as much as she needs him to. He is more concerned about his material wealth and is filled with hatred and bitterness towards Christians because of the way he has been discriminated against by them.
One can sympathize with Jessica, for it seems as if her father does not consider her at all. He is in full control of her life, and she rebels against his autocracy and greed. In the end, she feels that she has no choice but to leave his stifling influence and find happiness in the arms of her true love, Lorenzo.
Shakespeare's portrayal does not seem ambivalent and is rather compassionate towards Jessica. The only damnation for what Jessica has done comes from her father. No one else criticizes her behavior, and Shakespeare's presentation of Shylock makes him more a villain than a victim. In the end, viewers are inclined to applaud Jessica's actions rather than condemn what she did.