I knew just where to take her...
My sis orders drinks and the slice for us, I order chicken salads for us both and a couple of kids packs for my boys. They come with drink, snacks, fruit and a toasted sandwich. They even have a special delicious gluten free version (but my GF daughter is away today so standard ham and cheese toastie will do).
A lovely motherly staff member comes up and asks if he's OK, suggests kindly that I take him outside to calm him down while she gets our table all sorted.
Eventually the screams turn to giggles and he runs off to chase his brother and play on the slide.
55 Princes Street