For an 8 year old, just lower the arm rest to seat level, then place one of those stadium seat cushions over the cup hold and across the arm rest.  Might need to air up the seat so its about level with cup holder when your sitting in it.  Started out like that when my kids were small, and still using it (have had the tractor 12 years now).  The nice thing with this, is that when they aren't riding with you, throw the stadium seat cushion behind the seat and have your arm rest and cup holder back.