Help with tractor question, new at this and need advice

So I bought a tractor for our girls, no time to build our own. In the 'upstairs' do I need pine shavings on the floor or just in the boxes? My problem is the floor isn't solid, there's a slight gap between the boards for ventilation so pine shavings will just fall through and blow away, help! The gap seems bigger than what's on their site so ill,be totally wasting the pine shavings

You can see the upstairs here http://thechickentractorcompany.com/Boarding.html sorry I can't post a pic from my iPad

The 'downstairs' is a run area so I don't need anything down there... Thank you!!!