Now, my youngest is 11, so it has been a few years, but honestly I think I would just keep it to what they will experience. "Mommy has to go on a little trip for a few days. You will be taken care of by... I'll be back before you know it, and if you are good, I'll bring a present!" Don't mention what is happening while you are gone, and when you do get back, just say you strained your belly and are sick, so that's why you are sore and resting. There will be time enough later to be honest. At the moment they are really too young.