I'd never start anyone on a brass instrument before about 8yo and when the adult teeth have emerged. Before then, you can do a lot worse than a recorder, if they like a wind instrument... they can learn about tonguing and finger co-ordination, as well as reading music.
Paradoxically, though brass instruments look easy because they either have a slidey thing or just three valves, the embouchure requires the face muscles to be very very finely controlled, and TBH before about 8, a child's face muscles aren't really up to the job. Best to start when they are.