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As others have mentioned above, starting nodes in this way isn't really recommended....

However, to answer your question, you're looking for the TimerAction. It's a one-shot timer, that you can use for something like this:

from launch.actions import TimerAction
...
TimerAction(period=60.0,
            actions=[Node(...), Node(...)]),

If you're really wanting to handle this in launch, again not recommended, you could make a custom action that subscribes to you sensor data, and when it receives valid data to start a list of Actions.