Almost every project post-mortem names something that could have been settled in the kickoff. Spend the first session on the questions people are too polite to ask out loud.
A kickoff whose output lives in one person's notebook gets relitigated a month later. The board and its summary become the reference.
'We will decide together' is the answer that causes the most delay. Say who decides when the group is split.
People raise real risks more readily on a card than out loud in front of a sponsor. Collect them in silence before discussing.
Scope and non-scope, the definition of done, the decision-maker, dependencies, risks and how you will notice early that the plan is wrong. Status reporting can wait.
An hour is usually enough when the questions are on a board in advance and people have added their answers before the meeting starts.