Sean is a 10 year old boy named on What's Your Favorite Word? where his favorite word was "conserve". He has served as a background character elsewhere in the series.
Interview for WYFW
"Conserve" is my favorite word, because it's so important to the world. You can conserve water, wood, paper, plastics, and lots of other things. I conserve at home, I recycle, and I know three ways to conserve: Reduce, reuse, recycle. I call them the three "R"s. Conserving is my SO favorite word!
He appears to be a member of the WordGirl fan club
he is standing at the front of the line that Becky and Bob are waiting in, appearing to accompany an unidentified purple-jacketed lady (possibly a mother or aunt). He presumably rides the roller coaster off-screen since he's later gone when Becky and Bob reach the front.
He is sitting in the back row of the audience watching the Funbots concert. He is sitting two seats to the right of the Grey-shirted lady (who is on the upper right) between a man in a blue shirt and a blonde boy.
The man standing on Tim Botsford's left who has asian characteristics like Sean does is wearing a similarly designed shirt, except the colors are flipped. This may be his dad.