I know that two child actors originally played Ricky and David. Anyone know their names?

Joel Davis (David), Tommy Bernard (also David), and Henry Blair (Ricky).

I'd have to point out I've listened to the radio show and watched the TV show, and the radio show is far superior. The radio show blows away the stereotype of "all's nice" that the TV show could perpetuate. It's lively, has great character actors, and Ozzie and Harriet have superb voices for the radio.