Children seldom get a chance to count a larger number of objects, say between 30 and 60. By seeing and counting this many they become more able to estimate larger numbers and give rough approximations, rather than just give wild guesses.
Try similar exercises with heaps of coins, counters etc, dividing them up into fives or tens then working out the total. A really good idea is to collect change (pennies etc) sort them and guess how much there is of any one coin.
Try the two worksheets below to start you off.