If their young ask yourself the question what styles you like for them
Do you want straight trunks or trunks with movement
With movement you could wire the trunks now and bend them
Make sure the roots grow sideways in all directions instead of downwards to get a nice flair at the base and just let them grow and get fat trunks best in the ground or big flat wooden boxes
and train them for bonsai when the trunks are at least 2,5 inch in diameter