With Kids in the house, I can understand Moms concern. Emotionally, she is worried about her kids.
When we have visitors here with kids, my wife runs around picking up the ammo and dropping it all in the basket on my desk. She knows the ammo is pretty safe if it is not in the gun and the chances of one of the little ones setting off a live round is slim to none, but the Mommy Instinct kicks in. (Maybe she listened to some of the stories I told her about me being a kid? Who knows.)
I would not lie to my wife.
1. It puts the kid who does know in a bad situation. (Mom and Dad tell me not to lie, but Dad is not honest with Mom...)
2. When the wife does find out, be it now or in ten years..., you'll catch heck about it.
3. Find a place where the kids can not find it/get to it.
4. You would not like it if your wife did the same to you.
Hopefully the safe has arrived by now, or she knows and everything is cool.
My newbie mistake: I bought a pistol before I realized what ammo cost to keep it running. Thank the Good Lord that my wife was understanding. I overcame that problem by reloading. (It is not any cheaper, I just shoot a lot more.)