library(data.table)
mother_dt <- data.table(mother = c("Penny", "Penny", "Anya", "Sam", "Sam", "Sam"),
                 child = c("Violet", "Prudence", "Erika", "Jake", "Wolf", "Red"))
mother_dt [, children := .(list(unique(child))), by = mother]
mother_dt [, child := NULL]
mother_dt <- unique(mother_dt , by = "mother")

child_dt <- data.table(child = c("Violet", "Prudence", "Erika", "Jake", "Wolf", "Red"),
                             age = c(10, 8, 9, 6, 5, 2))

combined_dt <- mother_dt[, child_age := ifelse(child_dt$child %in% children,
                                                      .(list(unique(child_dt$age))), NA)