-what if Netherlands joined the Central powers (Germany, Austria) during WO I?
The Dutch army in 1914 wasn't as bad as it was 26 years later. The Germans might conquer that last bit of Belgium on the other side of the Yser river, but otherwise the Dutch will clog up the Belgian road network even more. A small contribution from a small army which will cost Germany a friendly neutral to import stuff through. Slightly earlier CP defeat due to a more effective British blockade if you ask me.
And Belgium is likely to get the territory it wanted from the Dutch (Limburg, Zeeuws Vlaanderen) which will give Belgium coal and direct access to the North Sea from Antwerp.
That said, I think it's unlikely that the Dutch will join since victory is far from certain (although Queen Wilhelmina was pro-German at the time).
-What if what if Netherlands joined the Entente (great britain, France) during WO I?
Possible if the Germans violate Dutch neutrality by invading through Dutch Limburg into Belgium like they intended. The Dutch army is tiny compared to the German army, but the Dutch do have the Water Line which can stop the Germans.
Again, Germany has lot itself a friendly neutral to import stuff through in this case and once the blockade starts to seriously hamper the German war effort, advances could be made toward the Ruhr Area. If the Entente manage to get to German soil, that will dispel that 'stab in the back' myth before it even gets started.