Can windshield washer fluid be used to deice small airplanes?
The short answer is, you should not use windshield wiper fluid. 

TKS fluid and Windshield wiper fluid are very different things first and most importantly windshield wiper fluid has no FAA approval for this kind of operation (hence the price difference). You can find a full list of de-ice fluid for ground deicing of small aircraft in table 1 of AC 135-17. AC 135-17 also provides the FAA's suggestions on small craft ground de-icing in general.

TKS fluid is not for deicing aircraft on the ground it is for in-flight fluid based FIKI systems and is pumped out of various ports and flows over the wings/surfaces. It prevents the build up of ice in flight. For ground deicing, you use FPD/ADF fluid which is a thick viscus fluid that is often sprayed on the aircraft. Ground based deicing is intended to last from the aircraft from the gate (or other static position) until airborne and thus the FIKI system can be used. it also may prevent the build up of things like snow and ice while taxing.

The reason your POH says little if anything about de-icing is because neither the Dakota or the Cessna you mention are certified for flight into known icing, thus if icing conditions prevail on the ground they more than likely prevail in the air and you are not going anywhere anyway. The typical use case of de-icing a small aircraft is getting it out for the day after outdoor storage over night during snow, freezing rain, or enough frost build up to be an issue. If the temp is warming and icing no longer prevails the aircraft may still be contaminated. In this case you would use FDS liquid to clear the surface (and possibly a brush for any heavy snow build up). Many airports will have this on hand at the FBO.  

You should avoid warm water as it can freeze again if the temperature is low enough. 

This question is worth reading over as well.