Took photos of the empty walls (and stitchedthem together to create a seamless background in Photoshop).
Imported the photos into Adobe Illustrator.
Designed the little critters and the grass etc with the photo as background. Then resized the vector elements to be printed out and used as cutting templates.
Sticky-backed flock paper would do as well. You could also try patterned/printed fabric pieces with fabric glue or wallpaper paste.
You might need a coat of varnish to keep things pristine.