With young children, the best thing you can do is to set the stage for later communication by addressing things as they come openly and calmly, and by being sexually communicative.
For instance, things like
- using the right names for genitals, rather than calling them "down there" or "daddy's thing",
- making sure not to instill shame, such as, if a child likes to take off clothes in public, or masturbate in shcool, letting them know those aren't good PLACES for those things, not that there is anything wrong with the acts themselves,
- answering questions sincerely when they're asked. For instance, if a child asks why you keep your bedroom door closed, or why you have breasts and they don't, instead of being dismissive and saying "it's a grownup thing." really try and explain why on their level. In other words, adults sometimes keep the door closed because they want privacy. You have breasts because when you start to grow up, all women develop them, etc.
- when they do ask questions, be calm, available and don't panic. if you don't know the answers, tell them you'll all go take a trip to the library.
All in all, you come to it mindfully, and aware that it needs to be discussed at their level and pace, which it sounds like you're already doing, hon.
(I hope things have gotten better with the depression, btw)