Repetition is the best way to reinforce the order of letters.
As students to stand in a circle. Point to one student and ask them to start by saying the first letter of the alphabet. The next student should say the next letter and so on until each student has had several turns saying the letter that falls on their turn. A more complicated step is to have the student say the letter and then say a word that starts with that letter. Take turns with your child with each of you saying a letter in turn.