There's no such thing as small parts, only small actors. Or something like that. Ty Simpkins, the child actor who appeared in Iron Man 3, will be reprising his role for Avengers 4 because it's an inter-connected universe and everyone needs to keep appearing in the films.
Simpkins played Harley Keener, a midwestern kid who finds Tony and his damaged suit hiding out in rural Tennessee, searching for the Mandarin.
Avengers: Infinity War is set to release May 4, 2018. The sequel follows on May 3, 2019.