Stewie vs. Maggie
Scenario 1: Maggie is the new girl at Hugs 'n Kisses Daycare, where Stewie is attending while Lois and Peter are trying to get back to the States. In this universe, Janet does not exist. Outside, doing usual baby things during playtime, Stewie and Maggie both spot a discarded gun by the seesaw. They give each other a death glare and start making their way towards it.
Scenario 2: Left together in a playpen during a Simpsons/Griffin visit, they get on each other's nerves to the point of hand-to-hand combat. No ray guns, no Ruperts, no toys that can be used as weapons.