I plan to use this activity to practice the August 2014 song, "Seek the Lord Early." I'll explain to the children that "seek" means to look earnestly for something, just like you do when you play hide-and-go-seek. This activity is just a variation on that game.
First, choose a reverent child to leave the room for a moment. If the child is young, send an adult to wait with him. While the child is out of the room, choose another child to be "it." This is the only child who will not sing. Bring in the first child and have the primary sing him the song that you're working on. See if he can spot the child who is not singing by the time the song is over. Stress to the other children that you are all working together. If the child figures out who is "it," then all of the children have won the game. If they want to be successful, they will all have to sing their best.
If you have a small primary, you might have the child who is "it" mouth the words but not sing out loud. This is extremely challenging and my Senior Primary tends to enjoy it.
If you have a larger primary, you can choose several children to be "it" and see how many the child can find before the song is over.
If your primary has too much difficulty with a particular song, it may not be possible to spot the one child who is purposefully not singing. In this case, you can use a visual cue, such as a necklace or a button which the child who is "it" will wear.