Teaching theory suggests whole-part-whole. That doesn’t mean you have to start with 39′ or 33′ hurdles, but you can and should start with the whole hurdle process at lower heights and other activities which improve dynamic flexibility and mobility and not cue them in context of the hurdling. Once they start clearing 33′ and 30′ respectively for boys and girls is when you can work on parts. You also have to teach the acceleration phase in hurdling as a whole concept as well (1st 3 hurdles). You can start teaching rhythm by using 18′ hurdles with appropriate spacing till they get the hang of 3 stepping.