15 year old:

Tony:
Harley Keener

Steve:
Miles Morales and Bucky's daughter

Natasha:
Gwen Stacy (and Michelle Jones (MJ))

Bruce:
Shuri

Clint:
Peter Parker and Ned Leeds

Pietro:
Cassie Lang

Wanda:
Michelle Jone (MJ)

Loki:
Ms Marvel

Thor:
Groot

Bucky:
Steve's daughter