 According to Monica Rushton, Jared Rushton (qv) (Billy),  David Moscow (qv) (little Josh) and Tom Hanks (qv) (big Josh)  were put in a room with a bunch of toys to play with. Having silly  string, they tried to use it to gross each other out and that is how  the silly string scene between Jared Rushton (qv) and  Tom Hanks (qv) appeared in the movie.