Are you sure you don't mean 423 divided by 9? Normally the bigger number is divided by the smaller. And, as it happens, 423 can be divided by 9 exactly: 423/9=47. 9 divides into 42 4 times (36) with 6 remainder; that makes 63 with the next digit, and 9 goes into 63 7 times.
To divide 9 by 423 is much harder, and I think that might be too much for a 9-year old, because it involves long division and decimals.