The fight was personal and I don't think Gambunta would help Naruto fight kids anyway.
Actually yeah he would have been over kill.... but what about the kid toads..
Also they may be kids but they are both strong.
Naruto didn't master the summoning jutsu, remember how he failed to summon bunta when they were against Orochimaru and Kabuto?
a very devastating overkill.....that's why
When fighting Sasuke at the Valley Of The End, why didn't Naruto summon Bunta?
Sasuke would have been restrained and dragged back to Konoha.
Naruto probably didn't have the time to make the seals or he didn't think of it. He was focused on getting Sasuke back and during that moment, hadn't he just recently started calling the toads then? I can't remember how much time there was between Naruto signing the contract and him fighting Sasuke but I don't think there was a lot of time between those two events.
So he either hadn't the time to create the seals or didn't think of it. Plus in the beginning he wasn't good at summoning them
He actually learned to summon gamabunta, he even used him against gaara. U_U
Sannin fight was after Gaara fight, Naruto failed to summon bunta and Jiraiya said right after ''As I thought, he still hasn't mastered summoning technique''. The word ''mastering'' means that you're able to do for example. a certain technique eveytime you want to, Naruto couldn't.
Did he ever fail to do it with Karuma chakra?
You know what this means btw, the fact that Naruto had such a powerful summon at that age means that he had a way way more powerful skill set than Sasuke, even though he had only half mastered his Bunta summon.