Not sure how old your kids are, but my mom tells a story about how when she was young she was snooping through the house looking for her gifts and her mother caught her. She was told "if you want all your gifts right now, you can have them. But you won't have anything to open on Christmas morning. Which do you want?"
Solved that particular issue...
My personal thoughts are to send them back. Get them something else. Tell them you were holding them for a friend and her children so HER children wouldn't snoop.
Or, just tell them you're holding them for a friend, then give them to them anyway from you instead of Santa.
I had a friend who got a dog for her son for Christmas. They told him they were keeping it for a friend to give to his wife on Christmas. They let their son take care of the dog while they had it to "see if he was responsible enough" for his own dog (had been asking for one for a while).
On Christmas morning, they tied a bow to the dog's collar and sent it into their son's room to wake him up.
I love that story. I know it's not really on-topic. =D