You know, I've been thinking about this for a while myself. I've always supposed Kathy's Kira sounded a lot like Mokey or Apple (from The Christmas Toy), but I could never figure out what Jim's Jen would sound like. He never did many child/adolescent characters. Harvey (from Emmet Otter) is the first one that comes to mind...but I really can't imagine Jen sounding like Harvey. That's a bit of a stretch.
As a matter of fact, I wonder what all of the other characters sounded like during filming. Dave's Fizzgig and Garthim Master? Frank's Chamberlain? Steve's Scientist--wait, we don't have to wonder about that one.