by nerman8r
If you look through the forums you should find more complete answers, but in a nutshell:1. They're not that different. A couple monsters are changed, the aesthetics of the monsters is changed, and I believe there's one change to the KoT rules. The new rule is basically better for gameplay, but you could always use the new rule even with the old edition. Oh, and one card is different, but if you have the old edition, you could always remove the card, and even if you don't, it won't come up often.
2. The monsters are interchangeable (since the difference between monsters is purely cosmetic) and at least one expansion crosses both games. The KoT Power Up and Halloween expansions don't work with KoNY, but the KoNY Power Expansion *does* work with KoT
3. KoT is simpler, so I assume it would be easier for children, but I don't know much about children, honestly.