It's funny that most people who complain about the Prequels say they wanted to see several movies with Vader just killing everything. Then when they show Vader killing Jedi kids, they're upset. If they hadn't included that, I guarantee people would ask where the kids were and why didn't they show him killing everyone.
It comes down to they only had two hours or two and half, whatever, so they had to cram everything into that time slot.
It wouldn't be as much of a test for his new disciple then. I don't think he doubted that Vader would kill all the adult Jedi, but to kill the Padawans as well, that's a step beyond. That would prove he's Sith. There's most likely more Force sensitive people around to use so it's no loss to Sidious.
It comes down to they only had two hours or two and half, whatever, so they had to cram everything into that time slot.
I feel like Palpatine would have been more likely to order against killing the Younglings, but to take out all their guardians at the temple, and then taking custody of them, himself, to turn into Acolytes, Inquisitors, Shadow Guards, Hands, and that ilk.
It wouldn't be as much of a test for his new disciple then. I don't think he doubted that Vader would kill all the adult Jedi, but to kill the Padawans as well, that's a step beyond. That would prove he's Sith. There's most likely more Force sensitive people around to use so it's no loss to Sidious.