specs wise they are about the same but the nabi is made for kids so it is going to be more thought out in that way than the kindle fire is. the parental controls are a bit better on the nabi and the nabi comes with an awesome free educational software for kids prek- grade 5!!
Have both and am ABSOLUTELY positive the difference is ANY Nabi tablet is highly inferior to the Kindle, no matter what features you think it has that make it better for your younger child. Do NOT waste your money with this thing, trust me.