The twins (Fred and George Weasley) are already familiar to viewers from "The Last Kingdom" (2020), where they appeared in several episodes. According to the "Harry Potter Fandom", Tristan had to leave regular school and switch to homeschooling because of filming the wizard boy series, balancing education with his budding career.
Ruari Spooner (Percy Weasley) is an actor with theater background. He doesn't have many TV roles under his belt, but HBO projects like this often turn young performers into stars. The guy clearly hit the jackpot – his career's just getting started.

Source:
@hbomax
Who played these characters before
For comparison: in the films, the Weasley twins were played by James and Oliver Phelps, Ginny by Bonnie Wright, and Percy by Chris Rankin. It's hard to imagine the franchise without the red-haired family, so casting attention was especially intense.
Source:
people.com
Other family roles
We already know that Ron Weasley will be played by Alastair Stout, and his mother Molly by Catherine Parkinson, familiar from "The IT Crowd". Who'll play Arthur Weasley and Charlie remains under wraps.About the series
First season filming kicked off in July at Warner Bros. Studios in Leavesden and will run through spring 2026. Release isn't expected until late next year at the earliest.HBO plans to release one season per book. This will let them explore details and storylines that didn't make it into the films. We already know Harry will be played by Dominic McLaughlin, Hermione by Arabella Stanton, and Dumbledore by John Lithgow. Earlier, we at zoomboola.com covered how actor Nick Frost will look as Hagrid in the "Harry Potter" series.