Jack-Jack short-circuits the electricity while in sparkler form. Any place that relies on electricity is subject to this if he is in this form.
When Edna runs out of cookies to give to the Jack-Jack clones, the clones merge back into a single Jack-Jack which then turns into his Demon Imp and chases her. Later in film, we see that the Demon Imp jumps on Bob and starts gnawing on his arm. Fortunately, Bob is unharmed because he is Nigh-Invulnerable. However, if Jack Jack tried that with Edna, the results would be far more devastating. Edna even mentions in that scene that she painfully learned the consequences of trying to use cookies to placate the baby.
