My 6-year-old got a 20" Specialized HotRock for Christmas that has a pulley-wheel chain guide on it to keep the chain from falling off the ring. It is REALLY noisy (rattles), and I'm sure it's inefficient. Any advice on replacements?

I like the old-school plastic flanges that go on some of the less expensive 6-spd bikes. I tried putting an old Ultegra front der on there, but the seat tube is too big. I'm looking for the $5 solution here...

Here's what his current guide looks like (wheels behind chainring):