Sarah,
you have clearly outlined an inquiry approach to rockets, using a useful prompt, with time for the children to wonder and explore.
The investigation aspect is straight forward enough and I like the starter question you have implied – does a string make a difference to where a balloon rocket goes?
There are further starter questions that naturally arise as “take the next steps”
What will happen if we use different shape balloons?
Does the balloon go further if there is more or less air in it?
To assess how the children’s science understanding is changing, ask them to explain their prediction. Their explanations will then be supported or refuted by the activity.