Food ideas for Harry Potter party ??

Broomsticks - Fairy Broomsticks Recipe | Food | Disney Family.com

Every flavour beans - Just ordinary jelly beans in a bowl but label them as Every Flavour beans

There are also lots of ideas on catch my party by looking at what other people have done Browse Parties - Harry Potter | Catch My Party

how about green jelly with sweets shaped like spiders etc in it. You can also get a shimmer spray to go on cakes which could go on other things.

This sounds like lots of fun!

Could you go for a general wizardy halloween type foods?

I always do a big halloween feast every year with lots of spooky food, I'll list a few below for you:

Finger sandwiches - jam sandwiches cut in finger width slices with a smudge of jam on the end as a bloody fingernail

Maybe some breadsticks as wands?

Eyeballs - Cherry tomato with the top cut off - fill with soft cheese for the white of the eye and add a small piece of black olive as the pupil (up to you to decide whether this is suitable for your age range)

Last halloween I also did gruesome punch which my kids loved. I used a carton of ocean spray cranberry and raspberry juice and mixed it with lemonade so it's a nice blood red colour. The gruesome bit was the frozen jelly hand floating in it! You make up red jelly and fill a clean rubber glove with it. Once the jelly is set bung the whole glove in the freezer then just before serving cut the glove off and plop your severed hand in the punch bowl! You can add some strawberry laces for veins although I found these lost their colour quite quickly.

Another very good visual desert I used was eyeball jelly. Get a tin of lychees and pop blueberries inside them for the pupil. Pop one or two in each bowl and fill with red jelly. My kids were very impressed but wouldn't eat the eyeballs, much to my husband's joy as he loves lychees

I'll put my thinking cap on and see if I can come up with any other savoury ideas for you.

This site's got some good ideas:

Coolest Child Birthday Party Ideas

Another site with some good ideas & recipes, not sure if it would suit your age group but you could swop things around
The Hogwarts Celebration: Hogwarts and Honeydukes Recipes

Upon entering the party, you could sort each guest into their houses by taking names & houses out of sorting hat. For a craft thing each child could make a wand. Let the children create an explosion! Simply put 1 tablespoon of baking soda in a cup, and then slowly add vinegar.
Game wise I guess you could do a version of pin the tail on the donkey - place the wand in Harry's hand or something similar.

Hope this helps

These biscuits are pretty cool

Harry Potter Biscuits « The Biscuits Blog

Non alcoholic butterbeer recipe