Will the tree's health be heavily impacted if they don't get their recommended hours of sunlight?
Each species has its own preferred conditions but any tree can manage with less than ideal conditions so there's a whole range of sunlight hours either side of ideal that will still be OK.
More shade usually = extended growth which is not ideal for bonsai but the trees will manage. More sun will produce shorter internodes and may even restrict growth which makes it easier to develop compact trees but more sun = faster drying which can be difficult to manage. I tried growing maples with no shade to see if it could be done. Extra watering was difficult and burnt leaves not ideal but it can be done. I've since gone back to shade in summer. Both trees and I are happier with that.
Also should seedlings with small leaves get less sun until they get into better health? Or will the sun help the tree heal?
Not quite clear what your concept is?
Seedlings are just small trees. They can cope with anything the larger ones can. Just need to be aware that seedlings are often in smaller containers so will dry out quicker than larger pots. Sun is only part of the issue.
If you can manage shade for part of the afternoon when heat is at its peak that may be enough. Here I erect shade cloth over the nursery around mid summer so the trees have light shade (35%) for most of the day but only for the hottest months.
Each area will have different needs. Hot, dry will require more protection than hot, humid. Dry, windy will require more shade and shelter than less windy areas.
Take heart that trees can cope with a range of conditions and just have a stab at what you can manage at your place.