I always felt like Richard's Baby Scooter voice was similar to the "Frank Welker" voice, which is the cute, gurgly voice he created for tons of cartoon characters (such as Glomer from the Punky Brewster cartoon), and which was subsequently adopted by several voice artists like Dave Coulier, Howie Mandel and Chris Sanders, the director of Lilo and Stitch and the title character's performer. It's a pretty common voice, easy to do (just about any kid on a playground can pull it off) and fits a baby character very well, so i can see why Richard would use it for Baby Scooter. It's interesting to note that Frank used the voice on Muppet Babies when he took over as Skeeter after Howie Mandel left the show. I doubt there's a connection between his useage of it on the cartoon and Richard's performance in the movie, which came out a year earlier, but it's still interesting.