I would be so grateful for any information/ideas that you can give
In Krakow you could take them to: parkwodny.pl
Once you are done with Krakow you could move on to Bialka Tat en.wikipedia.org/wiki/Bia%C5%82ka_Tatrza%C5%84ska
there are many things to do there for kids and there is a nice big lake 10-15 mins away. You can pop over to Slovakia, visit Zakopane there is a natural thermal pool in Bialka
Bania is a pleasant hotel to stay in for kids: hotelbania.rtrk.p enjoy
This is the Hotel we would normally stay in when we visit Bialka: podkotelnica.pl/en/hotel
Here is a link to info about the lake closeby alternativepoland.com/eng/index.php?option=com_content&view=article&id=52&Itemid=58