The English language is riddled with exceptions to rules, but children can still learn a few basic ones about vowels that usually work. Along with a knowledge of syllables this can aid their ability to read and spell new, simple new words. Studying syllables allows one to demonstrate that the five vowels, and 'y' appear far more often than other letters. In fact they may be the only letter in a syllable. All the other letters we call consonants.
Consonants mainly have one sound, with a few exceptions.
Vowels have two main ones, their long and short sounds. I demonstrated why these might be called 'long' and 'short' by running my fingers along a ruler as I said one of each type in a normal fashion. The children could do this too. Saying a long vowel always carried the fingers farther, and of course the long sound was easiest to remember since it was the letter's name. Except for L5, long vowel sounds tend to happen when two vowels occur in one syllable and the second is silent.