On the way to the swimming pool, open the refrigerator (or close it and open it again if Edna left it open). If you do so, you barely have time, but it's possible to pick up the glowing key, go back, get to the dungeon and enter the secret lab right before the "Gee, I'm hungry!" cutscene. That means you can get past the Purple Tentacle.

You're not out of the woods yet, though. For your first/second kid to enter the dungeon, your third kid had to push the gargoyle, which means they're in a place where Ed is going to capture them! You have to beat the game before that happens because, when Ed gets there, there will be an unskippable cutscene. Once again, you don't have much time, but it's possible.

This strategy saves about six seconds over ringing the doorbell with the third kid and using the "Oh, the doorbell!" cutscene to skip the Purple Tentacle.