Hi! I'm planning a 15 day trip from South America to Europe.

Personally, I wouldn't try to fit in more cities. You can easily spend a week in each of these cities, with little day trips around them if you want to get out of town. It obviously depends on your interests. For example, if you're interested in art, from Paris you can get to Auvers-sur-Oise, where Van Gogh lived the last two months of his life and is buried, or to Giverny to see Monet's house and gardens with lily ponds. Both charming French villages easily accessible by train in about an hour. Tons to do around Amsterdam too. What type of accommodation do you have? We rented a houseboat in Amsterdam on Airbnb - way more fun than a hotel and right downtown. We've rented apartments in Paris - again, that means you can enjoy shopping in the markets and at least have some meals at home. The best advice I could give you is to do lots of research around your areas of interest, and then find accommodation close by. Public transit is great in both cities, but nothing beats walking. Have fun!

I'd suggest spending some days in Belgium. It's a highly underrated country in my opinion. Bruges has a fairytale look with its medieval architecture, cobblestones, and canals. It's touristy but should be quieter in March, especially if you visit on weekdays. Brussels is an interesting city with a beautiful square (Grand Place), street murals, and fun museums like the Musical Instruments Museum, Belgian Comic Strip Center, Toy Museum, and Belgian Chocolate Village. Great food and beer too! You can easily move between these cities, Paris, and Amsterdam by train. 

Here's how I spent two days in Brussels.

Brussels and/or Antwerp is fun to do in between them. Cause you can take the train. Also Utrecht in the netherlands is really cool to see. Or cologne

You Bruges and Brussels without spending a lot of time traveling. Both are on the to Amsterdamand train would be easy

I have been to both Paris and Amsterdam and they are equally beautiful and interesting cities. Amsterdam is an easily walkable city. You can take the kids to the Anne Frank House (get tickets online first!) You can take a half day trip (tours available, we loved it!) to visit windmills in Zaanse Schans, wooden shoes museum in Marken and gouda cheese museum (samples to munch on) in the beautiful harbor town of Volendam. Between Amsterdam and Paris you can stop in Madurodam(The Hague) to walk amongst miniature versions of many landmarks and cities nearby. It is an interesting side stop. Bruges is a fantastic stop. it is a medieval  city, stop for french fries,(created in Belgium) waffles and chocolate. These will keep any teenager happy! In Paris there is so much to see it all depends on what you are hoping to see. You can definitely go up to the top of the Eiffel Tower, to be forewarned, it generally is a long wait. The louvre if you are interested in seeing a museum. I would go up to Sacré-Cœur Basilica even if seeing a church is not something you want to do as it takes you up through Montmartre a quaint area with artists creating portraits in the square and you can look over city at the top. Île de la Cité is also a nice place to walk around and visit  Notre Dame. Enjoy your trip!

Stopping off in Bruges, Antwerp, The Hague and/or Leiden  would be great stops on the way from Paris to Amsterdam.  Bruges, with its mediaeval centre and cobbled streets, is especially kid/teen friendly.

What are your main interests? Museums? Art? Architecture? Cities or also some country visits? Can you self drive, which will allow country visits. What is your approximate budget for this trip? There is a lot you could fit in in 15 days