Avengers: Endgame was a global phenomenon, which brought in a range of audiences while also remaining targeted towards young adults to children, further backed up by the films PG-13 age rating as anyone can watch it with parental guidance. The poster uses purple, it attracts the eye, especially for a younger audience. It uses both star power and the more known character of the marvel universe at the centre of the poster, Captain America for example is one of the most popular among children, therefore in the poster are very up front and in the foreground, whereas other important characters such as the protagonist Tony Stark and antagonist Thanos are represented as being vital to the story by their prominence on the poster, both being the largest, which is easier for a mainstream audience to understand.
The older side of the audience this poster is targeting uses an external method, a lot of teens and young adults grew up with these movies over the 11 years since its conception. The poster itself creates a feeling of nostalgia by using the wide range of characters that have appeared throughout the larger Marvel Cinematic Universe, revisiting these characters is a larger appeal to a mainstream audience then what would have been possible if this film was not apart of a franchise of films.
It is hard to disagree with your points here, as the poster does indeed use purple, however i will oppose the idea that this poster is aimed at children as in this poster there are clear codes such as the serious facial expressions of the characters and the lack of child characters make this film not as relatable to children as you think and the colour scheme which, as you mentioned, was purple, is darker than the average children's film and at a glance isn't quite as vibrant and suggests that perhaps the film is darker than you may think (despite the purple). And why he ourple?