Conditional fields based on structure level
This is not possible natively to have conditional fields based on the structure's level. For the same cases as yours, I'm also using different entry types.

Maybe you could consider moving your children to a Channel and link them to their parent using an Entry field + some routing, or using one of the plugins bellow to add a bit of hack, but I would not recommend it at all:


https://plugins.craftcms.com/conditions
https://plugins.craftcms.com/cp-css