It's not like they couldn't have gone to sell hot dogs at the recital without Dawn there, or even set up their cart at Dawn's soccer games. Or even just found another venue to set it up at.
"Our cows make a decent living wage, so please, no tipping."
Out of universe, why didn't the writers have the kids keep up the hot dog cart?
According to TV Tropes:
http://tvtropes.org/pmwiki/pmwiki.php/Main/StatusQuoIsGod
In the story they might have given up the cart because they were too unimaginative to find another venue.
Or maybe they found out that they needed permits or something.
Or maybe they got tired of the work involved.
Also, I think that most episodes of episodic TV shows like this happen in their own separate alternate universes, different from the universes of the other episodes. So if all the episodes, except for a few that are sequels to earlier episodes, happen in alternate universes, the boys could have kept up the hot dog cart after this episode but we never see it in later episodes because those episodes are in alternate universes where this episode never happened and the boys never started the hot dog cart.