Under the Mountain (1981)

Teenage twin siblings, Rachel and Theo, on a summer vacation in Auckland visiting their aunt and uncle, meet a certain Mr. Jones, a mysterious man who helped find them when they got lost in the woods eight years earlier. Mr. Jones turns out to be an alien fighter from a distant planet who wants their help in fighting an alien giant slug family of eight members led by a certain Mr. Wilberforce who have arrived on Earth with plans for world domination.