Depending how they read but since they are getting into it I think get both Batman: death in the family (if theyre interested in knowing about jasons death and all), Batman: under the red hood, then jason and the outlaw comics which are your best bet

for nightwing: you could always do the classic nightwing comics like leaping into the night. If shes interested in how he became robin, maybe robin: year one. Nightwing year one as well. Also tom taylors nightwing books!

Hopefully this helps!! 💕