Super Babies!
Throughout The Incredibles and the first half of the sequel, Bob and Helen Parr have zero clue that Jack-Jack has superpowers. How did they come to this conclusion? Simple: Jack-Jack doesn’t display any supernatural abilities, like becoming invisible, running at the speed of light, possessing superpower strength, or unnatural stretchiness.
So, does that mean that Violet and Dash showed their superpowers while they were still in diapers? Logic dictates that that would be the case since nobody questioned the siblings’ superpowers at any time. How hard would it have been for Mr. Incredible and Elastigirl to raise two super babies?