Some of us might think that we need to wait until a certain age to start bible memorization with our children. It's not necessarily true.
Whilst every child and age are different, I truly believe it is never too early (or to late) to introduce them to the Word of God. Plant the seeds in their heart.
My son was 3 when I started. My daughter 2.
Choose a verse and share it with your children, read it to them and if they are happy to repeat it, the Holy Spirit is at work!
Sometimes, they will not want to repeat it and that is fine. Just read it for them.
The bible says in Isaiah 55:11: "so is my word that goes out from my mouth: It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it."
We are called to share the word with our children. Day in and day out.
May God bless you in this journey!