If you unsure take it to the vet
If you can't get them to take you, then see if you can get it to throw up.
Not sure how fast a cats digestion is but get it out quick
Rat poison is an anticoagulant, it basically causes the blood in the rat to thin and bleed internally and some times blood can run out of the ears,nose,eyes,mouth
These are symptoms to look for to confirm whether or not your car has ingested any poison
Still a vet is the best option, tell your parents that if your cat dies in the night that you'll blame them and the fault will be entirely on them