Hi Ms. Nikkimom,
Good for you. You're on the right track, and have your cuckold husband on the right track as well, it sounds like.
If you try to keep your girls in the dark about the nature of your lifestyle, that will be in accordance with the longstanding tradition of parents pretending to their children that sex doesn't exist, or it's a deep dark secret they're too young to know anything about. In reality, this widespread practice persists because parents find sex embarrassing and awkward to talk about. It does maturing children a great disservice, and is an unfortunate way to treat one of the most important aspects of life. It leaves children to figure things out for themselves, which they always do, but often in ways that are unhealthy or just plain wrong.
Children.... like married women.... usually know what's going on in their own households, so there's a good chance your 16 year-old daughter (at least) is aware you date other men. Since she is beyond puberty and may be dating boys herself — or certainly thinking about it — I suggest taking some time to talk with her, in private, about the nature of your and your man's "cuckold husband / hotwife" form of marriage. A way to do it would be to take her out for a long drive, with just the two of you in the car, during which both of you leave your cell phones and other electronic devices at home. And, during this conversation I suggest emphasizing that you love your husband, and he loves you, while being a "hotwife" satisfies your need for broader sexual and associated psychological experiences of a more intense nature, while your husband — who is fully in agreement with this style of marriage — provides you with emotional support, and finds it increases his attraction to you (i.e., it turns him on too).
Since you've said your 16 year-old was sired by a previous husband and does not get along well with your current husband, I suggest emphasizing to her that under no circumstance should she use this knowledge to attempt to taunt him, and under no circumstance should she talk with other people about this, because people generally tend to disapprove of this sort of thing. Rather, it is a form of marriage you and your husband have privately agreed to and find very satisfying.
Your 9 year-old daughter may be too young, as yet, to talk with about your "cuckold husband / hotwife" form of marriage. If this is your feeling, a way to handle it would be to stress to your 16 year-old that she should not talk with her younger sister about this because.... well.... she's too young. Rather, you will do that when the time is right.
Another way to handle it would be for you take your younger daughter out for two long drives, just the two of you, sans cell phones, and during the first drive talk with her about her upcoming puberty, sex, and relationships with boys in a broad sense. Then, during the second drive, talk with her about your and your husband's "cuckold husband / hotwife" form of marriage, as with your 16 year-old, but in an age-adjusted way. In her case also, you should stress (of course) the importance of not talking with others about the nature of your marriage. If you think she'll be unable to handle this requirement, that would argue for waiting until she's older for these conversations.
—Custer