Presidents have often used family members as untitled unpaid advisers. If I'm not mistaken, Franklin Roosevelt used his oldest son, James, as an unofficial adviser and assistant. Eisenhower's son was a WH advisor (though in a paid position), and his brother was an important unofficial advisor as well. So far as I can tell, it appears legal. So maybe she'll end up being the 21st century Edith Wilson, Id rather have Ivanka as president than her father. This is minor in the face of the things Trump is trying to do or plans to do. Presidents are going to rely on the advice of their close friends and family, so I don't really see the problem giving them office space. And making them go through the security clearance process properly.
Besides, it's not like anyone's using most of the West Wing these days. May as well sublet the place. Hell, she could move in at the state department...I understand whole floors of their admin building are empty, waiting for the President to appoint folks and waiting for their budgets to get cut to the bone. With the Trump WH being such a bizarre disaster, I can't really get that worked up about it. If Trump's not talking to Ivanka, he'll be talking to some ex-Nazi conspiracy theorist.