Good grief. I don't have children but I have encountered them on occasion :-) and I cannot begin to imagine how cranky a room full of tired, hungry three-year-olds might be. Kids that age (and older) need to eat every couple of hours - missing a meal and being out till 9pm just sounds crazy. If it were an adult party to which children allowed to come along, and the hosts didn't have children I'd not be too surprised as if you haven't experienced children you might have no idea how often they need to eat and sleep. But to have this arrangement at a children's party is odd and probably rude, I think.
Every time I am around young children I am just dumbfounded by the amount of eating and sleeping that goes on! Even when a friend visited me last year with her 9- and 11-year-old, the days were just organised around food! Then another friend came with her 2.5-year-old and we could barely leave the house without having to rush back for naps/food. As someone who eats and sleeps pretty much when I feel like it, sometimes not at all, sometimes loads, I just found it baffling! But now I know - kids need to eat and sleep, it's pretty natural :-)