I'll buck the trend here and say it's a bad idea to give takebacks if the blundering player intends to play in tournaments at any point in the future.
There are two reasons for this.
First, having to finish a blundered game brings home the pain of that blunder more clearly, and makes it more likely that the player will think ahead next time.
But even more importantly, a blunder isn't the same thing as a loss. Especially in games between young kids, it's quite possible that both sides will make several blunders during a game. Perhaps you make a terrible move and lose your queen. A few moves later your opponent may make a terrible move and lose his queen. And suddenly the game is reasonably winnable again.
Until you're at grandmaster level, there's no such thing as a game not worth finishing. The other player could blunder; he could run out of time; he could finish with a king and two bishops and not know how to checkmate that way. Learning to finish the game you start, even if it seems hopeless, is very important at lower levels of chess skill.