I shall deal with this in point form.
1. "...her smiling lips remain sealed." Let"s get the quotation right.
2. "Prod" means "poke with a finger or pointed object". The passage is about one teenager ("I") trying to ask another teenager ("she") who she has fallen in love with. She won't tell, even if "I" patiently keep her company while she eats cakes and pies while "I" just pretend to eat by "prodding" at the food with a fork.
I have to say that the use of "prod through" is not great here. We normally say "prod at the food" and it is quite a rude action. Frankly, to say the least, I think the use of "prod" here is highly unorthodox, though I understand that the author Jandy Nelson very well educated (Cornell and Brown) and a popular writer for teenagers. I really cannot honestly say it is the best English.
I seriously think that "plod" would be a better verb. "Plod" means "Work slowly and perseveringly at a dull task" - the dull task would be to keep the friend company by making or eating pies and cakes. Could it be a misprint in the book?
Next time when you quote something, quote it correctly, and state the author and the name of the book (this time it is "The Sky Is Everywhere")