There seems to be a simple Hollywood rule that if you’re doing an action film for kids, then the heroes have to be kids, too. If you’re doing comedy, you can get away with having adult protagonists (e.g. Looney Tunes: Back in Action, Scooby-Doo 2), and if you’re doing animation you can get away with almost anything; but if it’s adventure you’re shooting for, then Thou Shalt Cast Kids. Hence the side-lining of real Thunderbirds action in favour of a pretty weak kid-brother-saves-the-day story. Yawn.
(This might have been cool if it had been handled in a post-modern, Gen-X ironic fashion, but that wouldn’t have pulled in the core audience (kids). As it stands, the studio went for box office bucks over credibility with a niche nostalgia market. Tough choice….)