I got a graham cracker box from the recycling and turned it inside out since our washable paint won't cover well enough. I have learned enough in the past two years to tape it with masking tape since paint doesn't stick to other kinds of tape and glue takes too long.
Chiquita and I started painting the box red.
Honestly she got bored after a few swipes on a few sides and started painting on all the paper under the box and then was done. I wasn't worried though, as long as she participates in the making she's happy to tell me she made it all by herself, and that's good enough for me.
After the paint dried I cut out a ladder, windows, wheels and a hose. I bet you could find templates online that would look better but I just guessed and freehanded it. Chiquita helped me glue everything on and then she turned on the "siren" and "whoo-whoo"ed it all over the room. Success!