i'm living in chapelizod, having lived in castleknock for 3 years.
both areas suffer from heavy traffic, castleknock especially so, but it has the advantage of having a train link to town which is handy, the 37 and 39 bus go into town also. there's a good range of shops, tescos in roselawn, dunnes stores and M&S in the blanch centre. there's also a load of schools in castleknock for the kid.
there aren't actually too many family homes to rent in chapelizod, at least not on daft, one word of advice avoid the steeples area in chapelizod, anti social behaviour is a problem there.
chapelizod has a few pubs but no restaurants, you'll need to go into town for them or to castleknock. the park is close but forget about walking or cycling up knockmaroon hill, way too steep
if it was me i'd think castleknock would just about edge it for family living, you'll certainly have far greater choice for rentals, and don't pay the asking rent either, knock about 10% off unless it's a place that really fits the bill.
good luck with whatever you choose.