When do you start teaching your children about Islam?

The day they are born.

They won't know what you're talking about, but it will give you the confidence to speak to them consistently, and once they do get old enough to understand you, you will be a pro at talking to them in sha Allah.

Remember, keep it simple and easily digestible for the young kids.

Mention one point each day. Start slow, and continue to build.

Remember, you don't have to be a scholar, but just follow the Sunnah of the Prophet ﷺ who said:

Convey from me, even a single verse.

Sahih Bukhari 3461

It might sound crazy talking to a newborn baby, but it will make sense as they get older. You won't feel uncomfortable talking to them, because now it's an ingrained habit.

Small consistent habits like this will strengthen your ability to speak, especially about Islam. Your kids need it!