Fel didn't have his name set by the first thing that popped into Mukohda's head after forging his contract. There's no reason why that was the case with Dora and Gon. If they were dissatisfied by their name, the contract being mutual would suggest that if they agreed on a new name, they could change it accordingly. A mutual contract like Mukohda's is unprecedented, so there's not much of a reason to think that the conventional rules would apply to them. If nothing else, Dora and Gon could just refuse to follow Mukohda until he chose a better name for them, nullifying the contract, and only er-forging it when he was ready to give them a better name.
