It's happened to me. I promise you a 6 year old has no clue. So no worries. When it happens to me I simply say don't touch that! It's a back massager or it's a foot massager. That's all I've had to say so far and they lose interest. You don't have to explain it's a toy at all. Even if you did say "these are mommy's toys don't ever touch!" That will only pique her curiosity. I'm telling you, our kids have found mine and each time I say that's a foot massager or back massager, there ready to put it down. I told them once that I use it in between my toes when they are sore and they were grossed out! Lol.
Make it simple and try to make it short. Ours have never asked any more questions after I explain it's for feet/back. But mine are kept in my drawers or somewhere private, not in the open. We've been teaching the kids to respect people's privacy and when one of them had a terrible habit recently of digging through my drawers I had to be very firm. I also had to be more creative in hiding. He just turned 12 so there's no excuse for digging through my things. At a young age, respect for mom's (and other's) privacy should be taught. It has helped keep this from happening a lot now! I simply explained to them that they had no business just randomly digging through my personal belongings and it seemed to keep happening so I'd have to be firm. Finally my partner and I had a talk with the 12 year old. I told him that he has to stop disrespecting my privacy unless he wants me to disrespect his. I also said, one day you're going to come across something you wish you hadn't! Lol. That scared him away some and my partner told him that it was an invasion of someone's privacy and yadda yadda. Never had problems since.
We care for a 6 year old too and she completely understands not to dig. At that age they're much easier to fool though. You don't have to explain yourself because they'll believe you if you say it's a flashlight. We've never had her come across toys, but she's heard the speech "respect my privacy, I respect yours." And she has no problem with it... unless it comes to my make-up and nail polish boxes! Lol.
Just try a quick "foot massager" explanation. The less interesting you make it sound, the less likely they'll bring it up again (such as in front of family members! Lol) Then try telling her you like having your privacy and if somethign is not in the open then it should be left alone.