Well, if they were all girls there wouldn't be any... Oh, never mind.
5 seconds sound clip from the It's a Wonderful Life (1946) movie soundboard.
You can hear this line at 00:14:28 in the Blu-ray version of the movie.
Quote context
[...]
- You're shaking the house down. Stop it.
- Let them alone. I wish I was up there with them.
- But Harry will tear his dinner suit. George!
- That's why all children should be girls.
- Well, if they were all girls there wouldn't be any... Oh, never mind.
- Come down to dinner this minute.
- Everything's getting cold, and you know how long we've been waiting for you.
- We got a present for you, Pop. There you are.
[...]