Mayim Rabim

That's beautiful! I can't think of a more accurate (and age-appropriate) answer.

agreed. but I doubt it'll be long before she asks 'what mitzvah'? How do people answer this month after month?

My neighbor calls it the Mommy Shiur. I am married almost 20 years with an almost 18 year old boy and an almost 16 year old girl and no one has ever asked me or hardly noticed when I go!

I think it's a good idea on general grounds to be able to simply say "I need to go someplace" without anything further being expected - I think it's healthy to establish boundaries so that Mommy doesn't need to answer to the kids where she is going or what she is doing when she goes out, unless of course there is some particular reason a kid expects her home at a given time. I don't think it's a good idea for kids to expect or think they are "owed" answers to these questions - I don't see this as a mikva problem per se, the mikva is just an area where one notices if one hasn't established that parents don't need to answer to kids on their whereabouts.
This comes up seemingly every month at the mikva - someone in the waiting room always has some story about what they "had to" tell their kids - this is my very strong opinion about this one!
Also, I think if one doesn't go out without reason except to the mikva - start doing so! It's a good idea! :)

I think it's important to tell people where you are going...in general......but maybe in this case, if your husband is home, you could just say to the kids - ask Tatty. Or if a babysitter is there, say, "if you have any problems, call Tatty".

I was discussing this with a woman I know, and if anyone asks her, she says she has to be an attendant for someone at the mikvah.

Frombeneath, that only works if it is actually true some of the time. Our mikva has several paid attendants so it could never work for us.

Now, if she said she's helping a kallah, then that's something else entirely............