Po-tay-toes conundrum

I'm reading my kids 'The Two Towers' at the moment. Now, since they were born, you simply cannot say the word potatoes in my kitchen without me saying: "Po-tay-toes... boil 'em, mash them, stick 'em in a stew!" so the kids have heard this a thousand times and now say it themselves. As we approached the scene where Sam was due to say this, I could see the kids getting visibly worked up with excitement.

The problem is... Sam does not say this in the book! It reads:

'Po – ta – toes,' said Sam. 'The Gaffer's delight, and rare good ballast for an empty belly.'

So, unable to disappoint them, I added the line back in, and they CHANTED it along with me, then fell over laughing themselves silly for like a minute. And then I continued on.

So the question I need answered is: One day they are going to discover this deception! Should I confess now, to avoid future disappointment, or leave them thinking that's how it really is?