Simpson and Williams struck either side of half time as Garforth recorded their third straight win of pre-season.
Garforth edged the first half and Adam Priestley will wonder how he didn't convert a few of his chances (thanks to the Ilkley keeper), before Simmo slotted home the opener.
Garforth made wholesale changes during the second half and were a constant threat.
Martin-Wyatt, Hunter, and Barker came close as the home side clung on, and Williams floated in a sublime second to put the game beyond the hosts.