'Chockers' (sometimes 'chockas' or 'chocka') is mostly Australian, New Zealand and British informal speech meaning 'completely full' or 'packed'. 'The car was chockers' just means there was no room left. It comes from the older naval expression 'chock-a-block', the same root that gives you 'chock-full'. It carries no hidden or offensive meaning and is not connected to drugs, alcohol or anything to worry about. If your child uses it they have likely picked it up from Australian or British TV, YouTube or gaming friends. Treat it as a colourful word for 'crowded'.