Supplies: a combination lock
Give the combination lock to a few kids to try and open the lock. Then have the kids shout out numbers for you to use. Make a big deal about trying to open the lock and play-up that the numbers they gave may be the right ones. Then say, of course the lock will not open because you don’t have the combination, only I have it. Then you do the correct combination and open the lock. Let the kids know this is like their heart. Jesus wants them to accept Him as their Savior but they have to open their heart and let him in. No one else can open their heart for them, not even the pastor, only they have the correct combination. Then lead the kids in prayer.