Mirka, I asked my 10yo granddaughter. She said most kids would use OMG instead of yikes. If that doesn't sit well (and it doesn't with me, especially for MG), would "Whoa" work? Her 8yo sister sometimes says, "Dang," with a tone of wonderment, for something like this.
The 10yo said her circle of friends would probably say "She's so perfect," in place of "goody two shoes."