From what sort of age would you assume a child in a queue is the customer?

[SarahAndQuack]

Today I promised my DD she could choose a pudding at M&S. We put a few other bits in the trolley too. When we got to the till I saw they didn't have daffodils there (they usually do), so I gave DD a £20 note and told her to start unloading the shopping as soon as there was space. She knows how to do this. I went to get daffodils from the flower stand. When I got back, someone had come along and put her trolley in front of DD's. DD says the woman didn't say anything to her, just walked round her with the trolley.

DD is six; she is quite small for her age. Would you have assumed a child that age wasn't in the queue?

Aw that’s mean! I’d have had to say something.

@Whatsthefrequencykenny - no, I didn't say anything to her as she'd just put a few items down, and I wondered (as per this thread) whether it was reasonable for her to have just assumed DD wasn't in the queue.