It depends on how observant your kids are -- when I was pregnant with my third, my first guessed almost immediately -- he had seen me with morning sickness with my second and when I was sick 3 times in 2 days about 2 or 3 weeks along, he came up to me and asked me excitedly if I was pregnant again, and as I will not lie to my kids, I told him yes. A 7 year old is not good at keeping secrets like that, so we told his younger sister too. I think given a choice, we would have waited 'til the end of the first trimester, but you'll need to play it by ear. For instance, if you do get a lot of morning sickness or something like that, it might worry your kid a lot if you don't tell them. When pregnant with my second, I was sick so much that my first asked if I was dying, so we told him right then why I was sick. It was close to the end of the first trimester, but not quite there.