Jesus offers an intentionally humorous illustration: when a child asks for something to eat, a parent would not give then something bad (or dangerous).
Bread was baked in a small round loaf, more like a dinner roll than a modern loaf of bread, so potentially it could be mistaken for a stone. Both fish and snakes have scales, so it is possible to confuse the two.
Luke 11:1-13 includes two of these analogies after his version of the Lord’s Prayer and the parable of the Visitor at Midnight. He adds a third substitution: a scorpion for an egg. This may seem strange, but Middle Eastern scorpions are small and resemble a bird’s egg when it is asleep. This is an even stronger metaphor than the first two since the scorpion is very deadly. The point is not “how could the two be confused” but “why would you do such a thing?”
In the illustration, the child asks for something good and necessary for their lives and even then worst parent has the sense to give them something edible (and hopefully healthy). If evil humans know how to give good things to their children, “how much more” will God, who is the ideal good Father, give good things to his children when they ask?
It is important to see the child is asking for some basic need, their daily bread (from Matthew 6:11). They are not asking for their wildest dreams, or to be wealthy and have a great car and gorgeous spouse, or to “have their boundaries expanded” as in the Prayer of Jabez. They are asking for their basic needs.
There is a responsibility on our part as well, we must ask if we expect to receive, we must knock if we expect the door to be opened.
Our theology shapes our prayer (McKnight, Sermon on the Mount, 247). What we believe about God shapes the way we pray to God. If our view of God is similar to a child’s view of Santa, then we will be very disappointed when our prayers are not answered. (“I asked for a pony and got pajamas instead.”) What happens when a child is disappointed by Santa year after year? They “grow up” and quit believing in him.
If our view of God is similar to a vending machine, we will be very disappointed when our prayers are not answered. If I do the right things (rituals, devotionals, etc), or do not commit too many sins, then God must answer my prayer with blessings, right? Think of those credit cards with some sort of a reward for spending. If I spend money and make my payments, they will give me money back at the end of the year, a “cash back bonus.” God does not really work like that.
Jesus describes God as the ultimate, good heavenly Father who wants the best for his children, even if those children do not understand what is best for them.