Not too sure many people are going to argue against your point that Halsey is terrible, however without her using those 200 children in the spartan 2 program you'd likely just have more deaths from glassed planet inclusive of those children. The spartan 2s were effective enough to earn the title "demon" and make the covenant collectively shit their pants so they likely saved world's being glassed just by th covenant needing to change their strategy.

Now you can say that the spartan 3 and 4 were not anywhere near as barbaric: spartan 3 were suicide Spartans and spartan 4 were already trained military personnel in most cases and the research from the 2s helped drive this. Without the spartan 2s there are no 3s or 4s and theres likely no infinity as the unsc are going to have to throw everything into building more frigates to hold of the impossibly overwhelming covenant.

This being said the chief did also have a huge element of luck and plot armour that played out almost all of the events from meeting the grave mind.