A poncho does it all-covers a pack and a kid, provides more coverage during blustery weather, and allows great breathability. The Sierra Designs Kids' Hurricane The Sierra Designs Hurricane Poncho is easy for kids and parents both.  It's waterproof breathable with taped seams and provides full coverage that can be worn over a backpack.  It pulls on easily with a zipper at the neck, reinforced grommets at the corners to function as a tarp, and fully waterproof-breathable Hurricane fabric repels wind, water, and snow. Snaps along the perimeter allow you to close it down over your kid when the weather's turns really gnarly, or open it up for a bit more flow-through.  Even if you know it's not going to rain on your trip it's always good to be prepared.  The Hurrican Ponch makes it easy.
 
Features:

Sizing:     Youth S/M: Front length: 30 1/2" Body width: 38 1/2"

               Youth L/XL: Front length: 34 1/2" Body width: 46 1/2"