When do you tell your children? How do I hide my symptoms without arousing suspicion?

My boyfriend has 2 kids, 10 & 12. I've been doing my best to hide my symptoms from them, but morning sickness is start up. How do you hide exhaustion, nausea, huge breasts (though they're probably less likely to notice those), and smell aversion from kids in the house? Is there a plausible cover story? Help!