I think the issue isnt that the OP is saying the little girl should get to keep all the shortbread for herself, but it sounds like shes lucky if she gets *any* of "her" shortbread at all. If someone visited me and brought me a box of chocolates, id be annoyed if i went to have some and found that most if not all of them had already been eaten.
I admit I also have a bit of an issue around children being treated as somehow less worthy of the consideration or regard that are awarded to adults and this sounds like one of those situations to me, because shes a child theres a lot of "she shouldnt be upset", "she should share" going on, but if i posted saying "my mother brings me a gift and then eats it all, last time i didnt get any!" im not sure the same advice would be given.
If the biscuits arent really for her, then dont lie and say they are. If they are for her, sure its nice if she shares them but they should ASK her, and especially they shouldnt be ripping straight into the unopened packet.