dunno if I can honestly. “Add spline
mesh component” allows me to only
attach a Static Mesh there, not
another “Child Actor”.
Spline Mesh Components allow you to deform an otherwise static mesh along a spline. (judging by your road, you know this well). They have absolutely nothing to do with Child Actors Components.
If you were hoping that you could use an entire Actor as a Spline Mesh Component and stretch it along a spline, then no, you cannot. That would never work and is not supported.
You could attach an actor to a spline, sure, but they will not deform. Only static meshes can do that thanks to the way Spline Mesh Component handles them.