We have not been to London yet (it's on our list for 2019), but we visited Amsterdam and Paris in one trip with our then 4 and 6 year old daughters in April 2016. We had 5 days in Amsterdam and 8 days in Paris.
I think it's a lot to fit in three major cities in two weeks with those ages. We have an eight-week trip for 2019 that we are planning, and we are trying to ensure we do not bounce around too much because that is hard with littles. And honestly, it short changes the areas because you don't have enough time. It takes one day to travel between each place, and you will also be dealing with jet lag, which causes some site seeing hours to be lost the first day or two. I recommend cutting it down to two cities.
My daughters both absolutely loved Amsterdam, with the younger in particular enjoying it more than Paris. It's a very small city and so easy to get around to see everything. We took the train to Harlaam for a day so I could visit the Corrie ten Boom house with my older daughter, and that day turned into one of our two favorite experiences. Haarlem is a beautiful old city, it had our favorite church we saw (Grote Kerk) and it was only a 15 minute train ride, if I recall correctly. We found so many cute little shops (came home with chocolate and cheese!), and if it hadn't rained, there was a nice windmill and playground we had been hoping to see. We also absolutely loved the tulips in Keukenhoff, which is the main reason we chose to visit Amsterdam. It was magnificent and a very bright memory for both my girls still. I think they will be over by late May, but you can sign up for email alerts since it really depends year by year when they are blooming. Our girls also loved the library (this was a surprise hit that was very fun!), walking around the town, the boat tour of the canal, and my 6 year old absolutely loved the Van Gogh museum. We felt we didn't have nearly enough time in Amsterdam as we enjoyed it so much more than we had anticipated. I know we would all go back again in a heartbeat if we didn't have a million other places on our list we want to go! It was such a kid friendly city and truly was a beautiful place.
Paris had been a bucketlist wish for our 6 year old as well as for me, and it did not disappoint. It was rainy and cool, but that did not deter us at all. We had thought we would take some day trips outside of the city, but that never happened as we never quite finished everything else we wanted to do- there was so much! We got the museum pass and went to a museum each day. With little kids, even with one who loves art, after a few hours, they are done. The pass was great because we were fine just popping into one for an hour or two, then hitting another later in the day for another hour. The Musée de l'Orangerie was our family favorite. We saw the Mona Lisa at the Louvre and a few other items, and I highly recommend the Friday night hours for that museum as my 6 year old was able to sit on the floor and sketch the painting for a good 30 minutes since hardly anyone was there. We did a macaron challenge where we had to buy a macaron each from every place we passed that sold them, and we tried to find the best ones in Paris. It was an expensive but very fun experience that we still talk about! Playing with the boats at Luxembourg Gardens and Tuileries Garden was such a hit we did it twice in both places, although we preferred the one at Luxembourg Gardens due to the setting, nicer boats, and bigger pond. Our girls loved Sainte-Chapelle and Saint-Sulpice more than Notre Dame because they were not as busy. We went up the Eiffel Tower three times (get tickets ahead of your trip). My older daughter took a kids cooking class at Cook'n with Class in Montemarte that was great fun, while my husband and younger daughter explored around the neighborhood and rode the train.
So, given that we have not yet been to London, I recommend keeping Amsterdam and Paris and dropping London.