Most children, it may be true, can't act. However, less than 1% of the child population acts in films. Most child actors are exceptional beings, then they go off the rails as adults.

As for children being unnecessary for the story, I can think of very few more inaccurate statements. Here are a very small number of films in which children are vital to the plot and, furthermore, in which the acting is exemplary.

Au Revoir les Enfants (1987)
Fanny and Alexander (1982)
Le Souffle au Coeur(1971)
Les 400 Coups (1959)
Zero de Conduite (1933)
Ivan's Childhood (1962)
Europa Europa (1991)
Lord of the Flies (1963)
My Life as a Dog (1985)
Ratcatcher (1999)
The Go-Between (1970)
The War Zone (1999)