Goblins in this case can be viewed as devil's agents who force people to commit sins. Food items are presented as sins that man can get involved in if he doesn't have a strong will power. They are described in attractive terms (Bloom-down-cheek'd peaches,

Swart-headed mulberries,/Wild free-born cranberries (5-14)), just like sins and vices that initially appear very tempting but are eventually harmful to one's soul. In the very same way, these fruits look attractive and are tasty but gradually rob the body of its vigor and beauty.

Laura is a risk-taker and hence fell victim to a clever and tempting ploy. Lizzie is timid and conforms to the norms and thus could save herself and later her sister. This is a rather puritanical argument but that's how the author presents it. But there is another thing which is far more important than their risk-taking capabilities. It is the ability of Lizzie to use her willpower to stop herself from going "astray" while the lack of the same makes Laura a victim of a pre-determined...

But Laura did not completely lack free will. What she lacked was will power to heal herself. But she does have free will or she wouldn't have tried the fruits in the first place. This is strange and vicious "hide and seek" game that free will and fate seem to play with the humans. On the one hand, humans want to try their free will to take charge of their lives and on the other; fate continues to intervene in our lives.
Laura had the free will to taste the fruits; it was not fate that drove her to take this action. But it was fate that she ran into the goblins. Once the fate appears, it is up to man's free will to do as he pleases to make the most of a situation. This is the lesson we learn from this poem.

References

Rossetti, Christina. Goblin Market. New York: Franklin Watts, Inc., 1969.

Campbell, Elizabeth. "Of Mothers and Merchants: Female Economics in Christina Rossetti's 'Goblin Market.'" Victorian Studies: A Journal of the Humanities, Arts and Sciences 33 (1990): 393-410.