They did say that there is somewhere he can go if and when he gets fidgety. I assume the nanny is watching in carefully to whisk him away if he starts looking like he might get restless.
But like pp say - he's older now and been in school for a good while. He will be very much used to having to still and quiet for long assemblies and school events - type of school he is at these could easily be over an hour at times. Dd went to a private school, albeit not as posh as their school, and from the very start of reception they were expected to be part of lengthy assemblies and speech days, and to sit quietly and still, even on hard wood floors.