"Mom tells us to be careful when driving this car."
Translation:Ibu berkata agar kami berhati-hati ketika mengendarai mobil ini.
From my view as a native speaker, your statement written there will have the nuance of "My mom said that we had been careful at the time we drove this car". So it is like you are talking to your friends but they don't believe you were driving that car safely and then you shout "Tidak! Ibu bilang kami berhati-hati (kok) ketika mengendarai mobil ini". That "kok" is more likely to emphasize your plead :D. Hope it is answered!