Giving instructions in a logical order is very important, whether teaching music or another subject. Phrases such as 'you will be working in pairs', or 'you can choose an instrument' often divert attention from listening to any other instructions whilst their attention wanders to who they will work with or what instruments are available. A good strategy is to write down the sequence for yourself in note form so that you tell them what the task is and make sure they understand this, for example by choosing someone (e.g. from names on lolly sticks) to repeat the instructions back to you and the rest of the class, before giving instructions about who they are working with and the instruments available. If they are not totally clear with the instructions, go through it again as 30 seconds more input from you at this point saves time later when you realise that the instructions have not been clearly understood.
Make sure you also give clear time expectations about how long they have, for example: 'You have five minutes to develop two short repeating phrases that you could use in your own piece'. Children can work independently, even at a young age, if their time is well structured and there is a framework to help them re-focus attention.