help with converting

Hi there, need some help apologize if this is been ask before , am that tech minded.

My son has special needs loves reading, he got himself a Amazon fire, I tried many things online to convert books to read on it.

Have kindle 1.24 on pc , what books in saved folder I can read them there.

Gone from trying this Epubor, I put in plugin and tried convert to either moby or epub, but after it done try putting on the tablet, just keep getting warning invalid item, but from what people say it should show up as encrypted but nothing,

So could someone help me with this please, appreciate any help