Continuity mistake: After the Ballet, Alfalfa is being chased by the two bullies and Alfalfa is carrying a handkerchief that Darla gave him. He dives into someone's pool and swims across. When he gets out, he picked up his underwear but the handkerchief is gone. In the race, he has the handkerchief back.
Continuity mistake: When the kids run into the shed to change to their firefighter outfits to fight the clubhouse fire, it is glaringly obvious where they stopped the film and started it again: the sky has changed and is much more dim than when they entered.
Continuity mistake: During the big go cart race, the one of the tyres on the rascal's cart wobbles at times, but not at other times.
Answer: I tried looking this up on the IMDb and couldn't find anything about Robin Williams there. My only suggestion is to watch it again and see if there is a list of extras in the credits somewhere. Maybe that's all the part was, an extra.