The dog playing Laddie (the son of Lassie) is actually Pal - the dog who played the original Lassie in Lassie Come Home (1943).
This was the only Lassie movie in which June Lockhart starred. However, 13 years later, she would appear in her first of 207 episodes of the Lassie (1954) television series.
This film was the first shot in the Technicolor "monopack" process, where one magazine of film registered all three primary colors, rather than the original three-strip Technicolor process (introduced in 1932), where a separate magazine of film had to be exposed (and processed) for each of the three primary colors.
This film was first telecast in Seattle Saturday 15 June 1957 on KING (Channel 5) and in Philadelphia Sunday 16 June 1957 on WFIL (Channel 6); it first aired in Altoona PA 23 June 1957 on WFBG (Channel 10), in Chicago 7 July 1957 on WBBM (Channel 2), in Phoenix 21 July 1957 on KPHO (Channel 5), in Portland OR 3 August 1957 on KGW (Channel 8), in New York City 9 September 1957 on WCBS (Channel 2), in New Haven CT 26 October 1957 on WNHC (Channel 8), in Hartford CT 19 November 1957 on WHCT (Channel 18), in Fresno CA 1 December 1957 on KMJ (Channel 24), in San Francisco 2 February 1958 on KGO (Channel 7) and in Los Angeles 3 April 1958 on KTTV (Channel 11). Laddie finally found his way to Minneapolis 7 May 1959 on WMSP (Channel 9). At this time, color broadcasting was in its infancy, limited to only a small number of high rated programs, primarily on NBC and NBC affiliated stations, so these film showings were all still in B&W. Viewers were not offered the opportunity to see these films in their original Technicolor until several years later.