Diction is very important. Words must be heard clearly, vowel sounds are often harsh. This can be remedied by dropping the jaw and opening the mouth. Too often we try to sing through closed lips. Australians are notorious for this!
We also tend to slur the words, etc., I am becomes I yam; let us - lettuce, and I - andie (we want everyone alive and well) but I - butie. Have children practise in front of a mirror at home.
They will probably dissolve into giggles but keep persevering.