Well done Olaf!
Cut bananas show a growth burst at first,after cutting but this soon stops and they continue slower growth from then on till they unfold their first leaf. 20+C temperatures will help them grow faster after a few days but they do need light as well to help them produce a good first leaf. You can place them under artificial but strong preferably, light till the first leaf unfolds or even till warmer temperatures at which point the windowsil may be ok. They need to grow roots as well so growth won't necessarily be fast at first.
Enjoy them!!!! Looks like they are gonna be beauties! How tall fo you think this variety gets from what you saw during your trip?