Well - as others have already said, it is likely that the toaster is a big, catering one that would take ages to heat up, or that gets bits taken off and washed after the breakfast service is over, so would need reassembly before the toast could be made, or that might even be put away once breakfast is over - so it is not as simple as just making a slice of toast would be in my kitchen, for example, where the toaster is always there, and takes very little time to make a slice.
And I do know that some cafes will make toast at any hour of the day - but different cafes have different policies, kitchen set-ups, equipment etc etc.
Maybe the member of staff was on their own in the cafe, and couldn't leave the till for the time it would take to set up the toaster and make the toast.
Tbh - if the OP had come on and asked if she was being unreasonable to contact Sainsburys and suggest that their cafes change their policy so toast is available all day, I imagine she would have got a resounding YANBU - but complaining because they wouldn't make her pfb some toast, when it wasn't on the menu at that point in the day, and when this is not the sort of establishment where the staff will be encouraged to cook things off-menu just smacks of entitledness. As does her apparent refusal to contemplate any of the other items that were available, to nourish her child.