I'm flummoxed both by the assumption 'appetisers' must be shared or that they are somehow separate from the meal that so many posters have said would follow.
This may be because there's no precision about terminology. To me, the appetiser is something like the little bowl of olives and/or nuts that the restaurant puts on the table for you to nibble on while you consider the menu (and coincidentally to make you eat less, so they can serve smaller portions of the expensive food and make a bigger profit, but that may be just me being cynical). These are something quite separate from the meal itself and absolutely are for sharing with everyone at the table.
I put 'appetiser' in quotes above because what we appear to be talking about in this case is a starter course - that is, something you have to order at the same time you order your main course, but which is served separately first (yes, I'm dotting i's and crossing t's here, but I want to be very clear). I can't for the life of me see why that is automatically for sharing, unless previously agreed before the order goes in. DH and I may agree we'll share a portion of garlic bread or something similar, but that's part of our discussion of the menu. There's no presumption that we'll share, especially if we only order one because one of us doesn't want a starter. If that's the case here, and I think it is, then this isn't something separate from the child's meal but an integral part of it, and it *is* rude to take the plate away from her and share it out with those people who didn't, for whatever reason, order a starter. And that's why it puzzles me that so many people suggest she still had her meal to follow. She had her main course to follow, but part of her entire meal was shared, and not of her own volition.