Can anyone solve this problem?
No interpolated child hairs are created for parent/guide hair created in particle edit. Only parent/guide hair created with the Emission part of the particles panel can have interpolated children.
This is true for cycles, eevee, and all viewport displays. Hair particles created with Add and with Particle->Mirror have this problem.
Particle Edit view. The parent hair on the left is from the emission panel. The hair on the right is from Particle->Mirror.
No children and simple children produce hair on the right from the mirrored parents. Interpolated does not.
MonkeyHairTrouble.blend (1.2 MB)