Both have impressed and United boss Chris Hughton is now in talks over their futures at St James' Park.
"Marlon has gradually built on every performance to get stronger and fitter and his confidence is now soaring," Harewood's agent Phil Sproson told the Daily Mirror.
"Marlon and I appreciate he had a slow start at Newcastle due to injury, a shortened pre-season and lack of first-team football.
"But ideally he would like to stay at Newcastle until the summer at least."
Simpson is also hoping his current deal will be extended.
"At the moment, my last game is January 9 against Reading. I think there have been bits and pieces of talks between the two clubs, but there's nothing concrete," said Simpson.
"Everyone keeps saying that Man Utd are short of defenders at the moment, but a few are coming back soon and I've only got six months left on my contract there, so whether it would be the right thing to stay there or not, I don't know.
"But I'm happy here and if something can be sorted out, I'd like to stay. I haven't heard anything at all from Man Utd.
"My agent will start speaking to them soon and we'll find out what the situation is and take it from there with Newcastle .
"You don't want to be left in limbo. I'd like it to be sorted before the loan deal ends, but things like that don't happen straight away and it could take up until the end of January. I could go back to United and then come back here a couple of weeks later, I don't know, but I'd love it to be sorted out earlier as a nice Christmas and birthday present.
"Playing in front of 50,000 is amazing. At the other clubs I went to, we weren't getting those kind of crowds and with the players that are here and the quality there is, I want to be part of a team that gets us promoted."