Have fun painting your child's room and creating a pale green hill. Easy to do with a big sheet of cardboard and a pencil anchored to string. Figure out how you want your hill to look and then anchor the string at the bottom of the cardboard with a drawing pin. Take the pencil, and with the string taut, draw a smooth curved shape. Then with a craft knife cut out the semi circle so you end up with a stencil of sorts. The craft knife needs to be nice and sharp to create a smooth stencil, then use that to create a lovely rolling hill to add the stickers to. You can create overlapping rolling hills like in the picture for an extra special effect.
Then you and your child can have fun decorating the wall, creating a forest scene.