When children do their best learning is when we make it fun. They love to color, have them color by numbers or letters. They will recognize the letters or numbers and will not realize that it is learning.
Later take the little letter blocks. Start by scrambling them and let them find a letter when you call it out. When they find the letters pronounce the sound.
Your children will quickly be able to add letters to ‘at’ to make a cat, bat, hat along with other words. They learned without knowing it. When my granddaughter took her kindergarten test, she told her mom all they did was play games like we played at home. She did not know that it was a test.