TV's Frink said:

And let's face it, TPM was in no way his fault. Dude was ten. It's Lucas' job to either direct a good performance out of him, or hire someone better

I agree 100%. The acting in Phantom was laughably bad at times. The rest of the time it was just plain hard to watch. However, in a franchise where a single person has total control of every scene, every line, every camera angle - every single aspect , the onus is on that one person to make something worth watching. In the case of the Star Wars franchise post-Return, that one person is Lucas. To me, he failed miserably. Jake Lloyd, while being a terrible child actor reading equally terrible dialogue, was the least of the problems with that film.

That's where Lucas got lucky with the OT. He happened upon three leads who knew what they were doing. With Lloyd and Portman, he used two people who have zero depth. They're script readers who, to me, are incapable of emoting depth.

*edit*

I should point out that I've never seen the last two films, so I'm basing my criticisms of Portman on the one film only.