Is it a legal requirement for underage children to use an adult seatbelt? I know here in Sweden for example it is illegal to have child, under a certain height, in a car without proper child seatbelt/car seat. Form a legal point of view no seatbelt and adult seatbelt are more or less equivalent.

Here (Australia) children have to use a correct child restraint and/or seat up to the age of 7.

I believe the legal point is the same as you've stated - the law is "not correctly restrained" or something.