how to properly train young chess players

I have three kids aged ranged from 10 to 15. they can hold their own against their counterpart in our city. moving our however to regional and national competition, they have yet to make it. most of the time they placed at the middle pack.

I hope to help them improve further their craft and hopefully make it to the varsity team of their college schools.

I am asking it somebody could recommend an scheme or the proper way to help them improve further. i am not financially capable that is why i am hoping somebody woulc help me through this humble story of mine.

I will be very grateful. thanks