My daughter just turned 8. When she suggested Harry Potter as an idea for part of her party, we weren't surprised. She's been reading the series for the last year and a half, and though we hadn't seen the movies yet (at least not until right before the party), Harry Potter reigns supreme in this household. But, what started as an idea evolved into a theme - and into a way of life for us - at least for the few weeks leading up to the party!
For starters, we invited about 7 children, and planned for up to 15 to arrive at Hogwarts (our house) - which included siblings. We created the invitations on the computer, the concept was an owl carrying the "acceptance letter" to each child - inviting them to Hogwarts (our house). Each was printed on "aged" looking paper, then hand addressed by my daughter, and sealed with "potions" stickers (it was a real benefit that her birthday falls at the end of September, which allowed us to take full advantage of the early Halloween merchandising in the shops!) Here's the invite:

(if you'd like a downloadable copy of this, let me know & I can make it available!)
Over the next few weeks, we picked out costumes, convinced my mother-in-law and good friend to be teachers, my husband brushed up on his magic skills - in order to lead the Defense Against the Dark Arts class, of course, and my daughter and son set to work on decorating - rather, transforming - our house into Hogwarts!

(our house, transformed)

(our front door - the entrance to Hogwarts, via Platform 9 & 3/4s of course!)
The kids were welcomed to Hogwarts by Professor Sprout and... let's say, Harry Potter's grandfather. :)

Some kids arrived in costume, but some did not, so we had hats at the ready! As the children were waiting for everyone to arrive, we had the first Harry Potter movie playing - as a way to keep everyone in one place, and a way for any parents or kids unfamiliar with the story (huh?) to have an idea of what was to come!

(Chilaxing to Harry Potter & the Sorcerer's Stone)
Once everyone arrived, they were each given a Hogwarts Class Schedule:

(Hogwart's Daily Class Schedule)
Then, we encouraged each child to go to Gringott's Bank to retrieve their galleons, knuts, and sickles:

(Gringott's Wizarding Bank)
Everyone needed their magic money so they could buy their supplies in Diagon Alley. First the kids "bought" their broomsticks (again, birthday party around Halloween was a blessing here!), then a magical creature from Eeylops Owl Emporium:

(many magical creatures - supplied by our own shop, SeriousPlush.com)
And, finally, on to Ollivander's Wand Shop for the first organized activity of the party: wand decorating! My husband had purchased dowels of various widths & cut them to a variety of lengths, then sanded the ends smooth. Decorations included markers, stickers, googly eyes, and pipe cleaners:

(Choosing their own wands - or - did the wands choose them?)
Once everyone had their wands, I (as Minerva McGonagall, of course) officially "welcomed" everyone to Hogwarts and split the kids into 4 groups (we chose NOT to use a sorting hat or houses for this process, because we didn't want anyone to feel left out or that they were placed in the wrong house - say, Gryffindor instead of Slytherin, just for one wild example) of about 3 children each. We had four "classes", each led by a teacher.

(Snape's sister unfortunately didn't make it into this photo... I think she was breastfeeding Mandrake at that point)
1) I taught Transfiguration (each child received a cauldron of magical growth potion [warm water] into which they dropped dragon eggs [mini sponge animals - available at the dollar store!] and used their wands to transfigure them into magical creatures.

2) My mother-in-law (Pomona Sprout) taught Herbology - where she had each child put on a pair of earmuffs, transplant a baby mandrake (mums) into a pot, then apply dragon dung (potting soil), and unicorn blood (water).

3) My husband (Albus Dumbledore) taught Defense Against the Dark Arts - in which he put on a small magic show then taught each child their own magic trick.

(okay, the baby didn't really learn any magic tricks, but this photo was too cute not to include)
4) My dear friend (Severus Snape's sister, "Seraphina Snape") taught Potions - where each child had their own cauldron (mini mason jar) and first mixed baking soda & vinegar, and then, mashed mistletoe berries (blueberry juice) & Lethe river water (soda water). Yummmmm.

(Seraphina Snape & her assistant, Mandrake)
As each class was finished, the group would rotate to the next, until every student had a chance to attend each class. Then came lunch & desert!

(Vegan cupcakes with lightning bolts!)
And finally, once everyone had a chance to attend all the classes they wanted, we all went outside to play QUIDDITCH! Truth be told, this was the highlight of the party - the kids ran around the back field, creating their own game & rules - batting around foam balls with their broomsticks and having a grand time. The adults maybe had even more fun than the kids - watching their imaginations come alive was really awesome!

Once the snitch had been lost (honestly! It was lost in a neighbor's backyard & we couldn't retrieve it!), everyone came back inside for treats:

(treats from Honeydukes - including Bertie Bott's Every Flavor Beans - YUCK!)
When all was said and done, the kids were tired and happy, and the adults were exhausted! I had a blast planning and acting and "teaching". And... our efforts were well rewarded by my daughter's assertion that it was the BEST day. (yay!!!! forever memories, created!)
PS: Writing this post had me reliving our first kids' "theme" party and I'm just so glad I got to share this with you. If you ever have (or had) the opportunity to host a Harry Potter party, let me know. I'd love to see!!
PPS: We've been watching the movies in order now - we're finished with the second so far; I'm a bigger Harry Potter fan now than ever before. I'm still reading book four. My 8 year old has graciously agreed to stop reading after book 6 to... let me catch up. ;)
PPS: Spell check had a field day with this post. Why in the world is quidditch not in the dictionary yet? ;)