find next(not necessarily immediate next) child of another parent in hierarchy with tag "checkpoint"?

there are 10 gameobjects in hierarchy
4 of them has a child with tag "checkpoint" and those child have box collider with trigger turned true
there is an empty gameobject called nextCheckpoint
when player ontriggerenters first child
nextCheckpoint's transform = next child with tag "checkpoint"s transform
if there is no next child in hierarchy with "checkpoint" tag,
nextCheckpoint's transform = first child with tag "checkpoint"s transform

hope im clear enough

i want to find next gameobject in hierarchy, not necessarily immediate next, parent or child, with a specific tag