While there is no halachic stricture against others knowing a woman is niddah, her status should generally be kept private for the purpose of modesty. Within the immediate family, there are two major approaches to this issue. Some couples make every effort to keep their children from knowing, while others give the children information on a level appropriate for them. We suggest that you and your husband discuss which is a better fit for you.
The more open approach is based on the idea that, since taharat hamishpachah is one of the foundations of the Jewish home, it can be constructive for children to have some awareness of it. Following that approach, you could answer your daughter's questions in a direct, factual, age-appropriate way.
For example, you could discuss how there are different times for different things. There are times for you and your husband to have your beds together, and times for them to be separate, and the Torah helps you know when.
You could also add that it's private. She can discuss it with you or her father when she wants, but it is not really something to bring up with other people.
Couples who want to keep the status change to niddah completely private can head off children's curiosity by always keeping the beds apart during the day and pushing them together at night when the wife is tehorah (or vice versa). This may be something to consider when your children are older and more sophisticated.
Behatzlachah!