I think the age should be taken into consideration, at some ages it would be far worse to host a party that starts past a traditional time for little ones to be out so as to make sure to be past dinner time.
With little ones there are other issues at hand it is considerate for the host to avoid having a party likely to interfere with work or school 6:30 seems late enough that most 8-5ers can be home and get the little one to the party with out too much rushing, and again it is considerate to try to have the party over in time so as to avoid fights over bedtime or curfews. I thank you would have to start at 8:00 to somewhat safely avoid dinner time and that likely seems pretty late for a week night kids party to start.
Likewise People esp. kids eat dinner at varying times for some 4:30-5:30 is pretty normal in which case the party is fine likewise if people work later hours 8:00-9:00 is not that uncommon (figure is someone works till 6 has a 30 minute drive to pick up kid 30 to get home you are at 7 dinner in an hour would be
Predicting when people eat dinner is really a tough one.
Also not everyone is as rigid about dinner for many it would be no big deal to grab fast food on the way to the party, and likewise you could have dessert first at the party and dinner later, re-warmed when you get home I see little wrong with either option.
She informed you of what is being served that is the most critical point, the kids will be fed something, and the guests are free to work around the time/food as needed. There is no reason someone can not eat sooner or latter on some occasions. Heck some people might love to ditch the kid with a quick burger then a party around dinner so the adults can have a nice dinner out.
I dont think it was rude (I think it is odd to have a kids B-day party during the week) there are too many other ways she could offend for me to hold this against the host, and what is being served is disclosed that to me is the most important thing.