Peter was keeping an eye on his nephews, watching the pair of them as they tossed snow back and forth. They were too intent on one another to notice him yet, which was fine by Peter. It gave him enough time to pull a few of his own snowballs together, using just a touch of cryokinesis to get the snow packed tighter, into a heavier projectile.
"Heads up!" was the only warning he gave, before starting to toss snowballs at Monty and Simon, and at the others starting to emerge from the front doors.