I've just been on holiday flying with Virgin. They did serve hot chocolate in paper cups without lids. They only half fill the fairly large cups.
There are a number of reasons why I wouldn't have let my DCs sit alone, especially this mix of age groups. I like to supervise with food. Even without hot drinks, those hot meal trays are a bit of a hazard when you are pulling off the foil lids. I can't imagine a 6 year old not struggling with this, and I wouldn't expect a slightly older child to have to take responsibility.
Also, Virgin flights have the personal entertainment TV things. There are two options on the menu, one is 'Just for Kids' and one with every option, for adults. There is nothing to stop a child selecting the adult option, my 9 year old did it by mistake. For April, one of the films on offer was apparently about a plane crash
, others had sex scenes - I would have wanted to leave a child unsupervised with this access.
Then there is the obvious one of emergencies - life jackets etc.
I can't see why, wherever you are on the plane, if you were awake you wouldn't have noticed the drinks trolley. If you are at the toilet they always get in your way. They are so slow coming down the aisle you are aware of them. As well as this they are a double row in depth so one steward would have been serving either the grandparents row or your own at the same time as the children's row.
I just don't get how the drink could have been served with no adult in the party noticing, unless you were all asleep, and if so of course YABU.