When your 4-year-old asks, "Mommy, why is your belly getting bigger?"
I told him it's because I've been eating a lot of food (which is true). We are not ready to tell the kids yet, but I'm not sure how much longer it will be until the big guys stops believing my food story
On the other hand, my friend's DD, who is 8 months older than DS1 and goes to daycare with my boys, didn't ask any questions about her mom's belly. They didn't tell her until after the anatomy scan at 18-20 weeks!