Start with a walk. You can intersperse running with walking. Make it informal. Set challenges and obstacles, such as short races between markers such as trees or telephone poles. Run a race together.
Many events have both adult and children's divisions.
Cultivate an outdoor life. Biking, hiking and just being outdoors will set the tone for running and general fitness. If any family member chooses not to run or cannot keep up with the group, give that person another job, such as holding a stopwatch at a track. Be a role model. Research shows that the No. 1 influence on children's fitness habits are those of their parents.