in R: take random sample of data.frame from and preform operation only on those

How would I go about getting a random sample from a lot of data.frames, so that I can perform some quality control just on my random sample?

Thanks in advance for your help and suggestions.

Here an example that hopefully clarifies what I am looking for:

set.seed(1234)        
i = 1        
for (i in 1:10){        
  df <- data.frame(x = sample(1:100, 5, replace = T), y = sample(1:100, 5, replace = T))         
  df <- df %>% mutate(z = x/y)        
  nam <- paste0("dataframe_", i)        
  assign(nam, df)        
}        

dataframe_1        

# x  y         qc        
# 1 28 38 0.73684211        
# 2 80 16 5.00000000        
# 3 22  4 5.50000000        
# 4  9 98 0.09183673        
# 5  5 86 0.05813953        

# random sample from my dfs        
randomtest<-sample(1:10,5)        
randomtest        
# [1]  5 10  8  9  1        

# QC         
df %>% mutate(qc = x/y)