Trumps kids worked in the administration directly for Trump. Had access to high level clearance. Used these positions to fly around the world on taxpayer's dime with secret service protection. They spent their time primarily building the Trump brand business relationships with Saudi Arabia, Turkey, Israel, China. All as representatives of the US government.
Hunter Biden is not employed by his father, is not working in the White House, does not have government clearances. He's a grown man with his own life and career. If he did something wrong, he should be charged, if he didn't he will move on.
Once a child steps into the political arena, he or she is fair game to be scrutinized. Hunter has not made that move.