# produce fruits that are worthy of repentance

Just as a plant is expected to produce fruit that is appropriate for that kind of plant, a person who says that he has repented is expected to live righteously. Alternate translation: "produce the fruit of righteousness that shows that you have repented"

# to say within yourselves

"saying to yourselves" or "thinking"

# We have Abraham for our father

"Abraham is our ancestor" or "We are Abraham's descendants." If it is unclear why they would say this, you may add the implied information: "so God will not punish us." 

# to raise up children for Abraham

"to create children for Abraham"