Children worship their parents by putting Tilak, offering flowers & bowing to the parents. Parents put Tilak on children & bless them. Experience the God residing inside one another. The blessings of parents bring goodness to the children.
Parents tell the stories of devotees of parents like Ganesha, Pundalik, Shravana Kumar etc.
Children can also perform some play on these devotees eg. Children can enact as Lord Ganesha to worship his/her parents as Shiva-Parvati.
If possible, do such event at a community hall where Posters depicting slogans of Parents Devotion can be put up.