如何删除数据框中孩子的孩子，以绘制R中的网络？

install.packages("igraph")
install.packages("tidyverse")

library(igraph)
library(tidyverse)

df <- data.frame(
  child = c("孩子A", "孩子B", "孩子C", "孩子D", "孩子E"),
  parent = c("父母A", "父母A", "父母B", "父母C", "父母D"),
  grandparent = c("祖父母A", "祖父母A", "祖父母B", "祖父母C", "祖父母D")
)

graph <- graph_from_data_frame(df, directed = TRUE)

graph <- delete_vertices(graph, V(graph)[name %in% df$grandparent])

plot(graph, vertex.label = V(graph)$name)