We told DD pretty early - well actually she said something about babies coming and it was just right - i think it was about 7-8 weeks.
She was just under 2.5 years at the time.
DSS was told after we got to 12 weeks, this was his request, we had lost a baby the previous year and he was really sad by it and said that he wanted to know when it was 'safer' so told him when everyone else found out (although a lot of people knew and had to keep the secret and surprisingly DD didn't say anything!) He was 12 at the time (now 13).

I think that it depends on the age of your child and how they will understand the information.