It’s a good idea to use flashcards to help your child learn sight words which are the building blocks of reading (and writing). However –
Use flashcards in addition to teaching sight words by using books you’re already reading with your child. Why?
We want the words to be meaningful to your child. When you point out a sight word such as “I” in a story you’re reading, your child sees how the word is used. It’s part of a story and has more meaning than if it’s isolated on a card.
When your child is about 4 years old, you can start introducing sight words, but it really depends on the child. Some children might be ready at 3 years, some at 5.