I think any of those months will work well as far as weather goes.
I've visited Paris, Netherlands and England multiple times in April and May. This last April I flew in to Amsterdam, spend days there, took the train to Paris, spend 10 nights there, took the train to London, spend nights there before meeting up with a tour of Northern England. I've also traveled to Paris and London in September.
For Paris next year, go ahead and take a look at hotels for April/May as prices are skyhigh the whole spring/summer/fall. I don't think the Olympics will have a huge impact on crowds in April/May but it has impacted prices. September is one of the very busy months in Paris as it is Paris Fashion week.
For London, any time will work.
I've hit heatwaves in both London and Paris in late August/early September so if you are not tolerant of heat I'd go later in the month.
I'd not really recommend taking a day trip to Haarlem although you can get a fast train from Paris to Amsterdam that takes about 3hours 30 minutes, then a local train out to Haarlem but that seems like a long way. What particularly draws you to Haarlem? If you want the "canal" experience there are closer "cute" towns....including Ghent and Bruges in Belgium and Colmar and Strasbourg in Eastern France. If you feel like you must visit Haarlem, I'd recommend booking an Open Jaw flight itinerary and flying In to Paris and out of Amsterdam or the opposite. You don't book this as 2 one-ways, you book on your airline website as "multi-city" or "multi-destination".