I think the children were outside the palace playing in the snow. I do not know if Anastasia was annoyed with Tatiana or if it was a harsh prank she played, but she rolled a rock inside the snowball and it hit Tatiana directly in the face. I don't know if Tatiana went unconscious, but it was definitely a big shock and she fell on the floor stunned. Anastasia hardly ever cried but, realising that she had possibly come very close to harming her sister, she completely burst out into tears. I don't know if any adults were with them or what happened after that.
Perhaps Holly could help!