I saw this warm-up in a video on Steve Smith's site www.tennissmith.com. Works for all ages and abilities.
Set up a cone, just beyond one doubles sideline. Players jog lightly in a line from the opposite
doubles sideline to the cone. As they reach the opposite sideline and head back to where they started, when the last player in line passes the cone (red arrow), that player sprints to the head of the line (green arrow).
Continue until all players have had a chance to be the sprinter.