You may not want to hear this, but I would just ignore it. It's an attempt at garnering attention, it sounds like. I have a couple of little boys who like to shriek, and it's a horrendous sound. I say firmly "We use INSIDE, QUIET voices". And proceed to give it nothing further. It may be a loud few days, but I have the feeling that they will figure out that screaming gives them nothing but a grumpy mommy/provider.