Ha, when I just saw the title I assumed that you meant an adult guest and I thought "well yes, that's a bit rude, normally one waits until food is offered, unless it's laid out with clear intent to be eaten" - but when I saw it was a 10yo girl, then no, YADNBU.
My children ask for snacks. If I let them loose on the food in the kitchen, they couldn't be trusted to stick to the healthier stuff, entirely too much sugar-based stuff would be consumed.
But my children don't eat constantly either. However, when friends come over, they do seem to be constantly on about how "hungry" they are - balls, they can't POSSIBLY be! but they come from homes that are on constant snack cycles. DS1 in particular has a friend who, on any afternoon, will be given fruit, rice wheels, disgusting noodle things and 2 ice poles (at least) at small intervals. So Ds1 gets into the habit too, even though he's not really hungry.
When we were small, the budget was very tight, so we had to ask for food/snacks if we wanted it - unless we wanted to pick our own (e.g. raspberries, strawberries, redcurrants), and quite often we were told No.
I remember once, when I was about 5, raiding my great grandmother's store cupboard and hiding a pack of mini-cheesy biscuits in my top - but was caught of course because I looked so GUILTY - and got a really big telling off and warning that I was stealing (which I technically was of course!) Never did it again. Always used manners and asked first; and that's what I expect from my children and from their friends too.
When they buy their own food, then they can eat it whenever they choose to without recourse to me. Til then, they ask first.