I'm gonna jump in here and say that the way Anthropic's KV cache works (prefix matching) doesnt guarantee this at all if the subagents are all put into flight at close to the same time. There's nothing in the agent SDK that allows you to instance more than one agent simultaneously, and the docs don't say anything special about KV cache use by agents in this scenario.

You could probably engineer your own harness or workflow to warm the cache by running a single warmup agent, but this is not at all how the tools behave out of the box. It's mostly a cost optimizaiton, so Anthropic really don't have any incentive to do it proactively, and if they did, surely they would want to tell their customers how much they were saving?