2 weeks to kill.....Where to go next?

Switzerland and Germany? That's on the way to Amsterdam.

Have you considered going south to Apuglia? It really. IMHO..is a different experience. I have travelled to beautiful Italy many times..and just last year made it to Puglia..solo. in June..very warm..the beaches are laid back..the water clear blue and warm. The other thought is the Dolomites..hiking..biking..easy walking..surrounded by beauty. Also this year heading to Calabria for 10 days..but that might be too remote for your kids..altho..more blue blue water! There are also National parks..and quite easy to get to Sicily. Perhaps this is too much Italy for you..
Each of those areas present a different region..culture..food..etc . Regardless..your trip sounds lovely..enjoy,!!

Belgium & Luxembourg? Also on the way.

If it’s your first time to Amsterdam you could easily spend more time there. Much to see and experience, many day trips to Haarlem, Delft, Edam, The Hague, etc. also a night or two in Bruges and Ghent. Last time we visited we stayed 9 days.

If you decide on Emilia-Romagna, you could go to Ravenna, see the Byzantine mosaics (really wonderful), and take a mosaics class as a family, at kokomosaico. You’d need to reserve on their website (kokomosaico.com). If you enjoy hiking and birdwatching, you could visit the Po Delta Park, just east of Ferrara, either with an organized tour or on your own, if you’re up for renting a car.

How about a short flight over to Budapest? Plenty of things to do there.

Add another vote for Amsterdam. A great time is easy to find there, even if you don't do the standard touristy thing.

Copenhagen! Munich? Salzburg? Bruges? Prague? Annecy? All fun!

How about more time in the Netherlands since you're ending in Amsterdam? Lots of interesting cities to explore with easy train connection to Amsterdam for your last 3 nights.

Rimini or Bologna or Pisa or Rome to Budapest starting at about $60 on Wizz or Ryan - Non-stop about 90 minutes.

Budapest to AMS starting around $135 on KLM - Non-stop about 2 hours.

Slovenia, then Austria. Tons of things for the kids: caves, lakes, castles, Sommerrodelbahn, water parks. In Slovenia, make Ljubljana your base. In Austria, stay around Salzburg. We stayed 5 nights at each with our teens in summer 2018 and had a blast.

Agreed with those suggesting Amsterdam. There’s easily 4-6 day’s worth of things to do depending on how thorough you want to get. Several museums, historic houses/churches/palaces, parks, and other things. Plus there are loads of day and half day trips to places like Delft, Haarlem, Rotterdam, Zaanse Schans, Enkhuisen, and more (I was able to combine the last two into a day trip when I went here).

I'm assuming all of these dates and order are already set? My initial impulse would have been to put the days toward Croatia.
But the Netherlands would be a good choice, as three days in Amsterdam will be packed, and there are plenty of options nearby--Delft, Leiden, Hague, Rotterdam.

If you do decide to depend the extra days in the Netherlands, you don’t need to stay all of those nights in expensive Amsterdam. You could stay somewhere like Utrecht, visit all the places listed above but also Otterlo for the Kroller-Muller museums (lots more van Goghs and more modern art, sculptures in the grounds) and the National park, the Hoge Veluwe where you can hike or hire bikes. There’s also the pretty towns of Zwolle, Deventer, Groningen, the Afsluitdijk which is the looong causeway that cuts through what was the Zuider Zee,. There used to be a museum/exhibition on the massive engineering work it took to create it. Lots to do in Holland outside Amsterdam, great though that is.