Regular task - Have a snowball fight between two sims.
Two unoccupied sims are needed for this task but age does not seem to be a factor. You will also need to build the snow park for this task, it is available to build from level 14.
Open the town map and go to the snow park in the west part of town. Call your 2 sims here with the whistle.
In front of the cabin there is convenient placed snowballs that you should use for this task. Make the first sim start with action Snowball fight then mark the other sim and click on the pile again and select option Join in.