The next time she does that, then as soon as you're alone with her (the same day), ask "Why do you keep doing that?"
"What's that?"
"Saying 'I love you' to me, but only when it's in front of the kids. Is this some sort of game you're playing, because I'm really not in the mood for games where our marriage is concerned. I find it childish."
Should be a good conversation-starter, if nothing else!
