well there is a reason they didn't want to play it that way, i can understand that reason. its mostly got to do with not wanting your country destroyed and your people homeless and hungry.
Might be a good thing for France and England to have a military alliance with The Dutch and Belgium, but not for those 2 parties. Because the fighting is going to take place there. And they all feared another trench war. And oh yeah, air power was way way more sophisticated. So burning cities is pretty much a guarantee. For Germany both countries were just a stepping stone into France. They didn't want revenge, they didn't want the resources, they didn't want the prestige.
Getting overrun like that was at the moment a blessing for both countries, unlike Poland. Except Rotterdam, that was unfortunate.
Most important thing was that the governments, royal families and gold all managed to get out(plus a lot of troops, military equipment and even some POWs). Plus the population wasn't overtly punished for the resistance.
I'm not saying being occupied by Nazis is better than trying your best to keep them out, its the opposite. But i highly doubt they will be able to keep both countries from being occupied. Plus they didn't know of course what an occupation by the Nazis meant yet, we do. Better have it with the least bloodshed and destruction. imho. I'd say OTL was the best outcome for both, considering they were going for neutrality from the getgo. Although maybe more for The Netherlands than Belgium. Then again Belgium was the theatre of WWI trench warfare.
France is a different matter though. France was a goal, not an obstacle. They wanted that prize. Let them work for it, a lot bigger country too so you have room to fight. Much better chances. Bgger army, bigger airforce. It'll cost them dearly.
I think that Belgium could have played up the armed neutrality stance by making a show of building defences against both Germany and France. They already had a big army so could argue that no matter who came, they were determined to make them pay.
Albert Canal balanced by defences on the French border, a central antitank defence line across the Hannut/Gembloux area adaptable to either direction (France has lots of tanks). They can't really get away with accidentally building them all vs Germany first, but there must be some scope for doing the bulk of the works so they can be used either way, leaving relatively minor work to place final barriers ( eg Cointet fences) once you know who's a real danger.
Also, if they're doing earthworks, it keeps the troops busy.
That and sending Gamelin to Martinique or Djibouti (so he can't spring the Breda plan at the last minute) should make the German army advance slower and losses higher in the north, reducing panic (a bit) when the southern breakthrough comes in.
That adds a few more days to weeks, more allied equipment destroyed in combat, sabotaged or evacuated and higher German losses. Now, they are two or three panzer divisions short vs OTL, their allies and occupation troops have worse equipment, they are many 10s of thousands of vehicles down vs OTL and Britain is in less of an invasion panic due to more equipment, closer to autumn weather etc. Italy may not join in.
Then you have to ask what happens to North Africa if Italy isn't involved? Does France have time to sign the joint sovereignty agreement with Britain? Maybe N Africa is British administered. Either way, Italy's navy is not available to the axis and the Med is more of less closed to Germany.
How does Barbarossa go with a vulnerable southern flank, no Italians and fewer vehicles and tanks?