I believe you'll find a few articles about this sort of thing at
http://www.dragonskull.co.uk/main_map.htm. I know Jim Magus and others in the Bizarre world have penned some insights on this sort of thing.
Look at the house and envision the parts of it that a child would touch the most, that's where you want "soft' wear marks i.e. where the top layer of the pain is thin and you see the foundation coat or an alternate layer of paint that was originally on the prop.
Strong coffee & tea will give you a quick & easy way to stain exposed wood areas so they look naturally aged. You may want to experiment however, using the same kind of wood (typically pine-based plywood for something like this but it could be birch, depending on the quality). What you want to do is find the right substance or combination as well as exposure time. There's a lot of variables but it really won't take all that long to narrow things down.
Brown shoe-polish (kiwi paste) is another major secret when it comes to aging things and giving metal parts a brass-like patina.
Last but not least. . . but only if you really want to have that "abandoned toy sat out in the shed" (or barn) for years. . . sit the thing outside and exposed to the elements for a couple of months before you start in with the other cosmetic accents noted above and by others.
