To expect my dd to have been fed & watered if she has been invited to....

[PestoBlizzardMonster]

A party from 2.15pm until 6.30pm?

DH picked her up and when she arrived home at 7.00pm I asked her what she had to eat, and she said they hadn't been given anything apart from a slice of birthday cake

When I asked why, she said it was because some of the other party-goers were staying on after the party for a sleep-over and the Mum had decided to feed them after the others had gone home

OP YANBU. Of course she should have been offered food, a couple of sandwiches, crisps or whatever. You're not asking for a cooked dinner or anything unreasonable.
We (society) tend to follow what's normal it heps us with our expectations and our way of doing things, this was quite off if you ask me.

When DD has a friend round after school they are generally collected between 6 and 6:30pm. DD usually eats her evening meal at 6:30-7pm with me and DH. However on playdates she and her friends eat between 5 and 5:30pm, so they bth get enough time to eat a meal before home tme. I would never thing of not feeding a visiting child when they weren't leaving until 6:30pm, esp on a school night.