Very simply tell the children you brought something to show them. Pull out your wallet and remove the two bills (the authentic $5 and the worthless $500). Let the children see the two. Then ask, "if I gave you a choice between these two, which one would you want?"
Even a 3-year-old will know to ask for the real money.
Ask why they chose the $5. Maybe ask, "Isn't the $500 worth more?"
Explain that one is real and the other is not. Then mention that in the passage this morning, Paul is going to point out that there is only one gospel.In verse 6 & 7 he writes: 6 I am astonished that you are so quickly deserting the one who called you to live in the grace of Christ and are turning to a different gospel— 7 which is really no gospel at all. (NASB)
Tell the children that just like with money, it is important that we know the true gospel message, so that we are not deceived by false teachers.
Pray for the children and have them return to their seats.