Ok. I had a harry potter party for my… 9th birthday party I think, so 9 years ago now! Which means it may be a little uncool but what I did was just get large pieces of cheap cotton material, one in red, yellow, green and blue. I then printed out the house crest for each Hogwarts house- I think we got them off the official website but maybe you could get the little guy to draw each house crest! Anyway so Gryffindor goes on red, Slytherin goes on green, Hufflepuff goes on yellow and Ravenclaw goes on blue. We just drew the curtains and hung them along the wall of the main room we were entertaining in, over the windows kind of.
We happened to have our family computer in the corner of that same room- just inside the door. So when kids came in they would sit at the computer and complete the sorting test to sort which house they were in- found on the official Harry Potter website (I’m not sure if it’s still there). Then they would get a party hat in the colour of their house. We had one central table and streamers in the house colours and stars hanging from the roof. We had serviettes and plates in house colours too I think.
My mum and sister did an awesome job with the cake- Just basically squares of cake covered in chocolate icing in in different sizes built up to look like a castle, then ice cream cones dipped in chocolate and turned upsidedown to become the turrets of the castle. I also had little Harry Potter lego figurines which they stuck onto the cake.
For games I had a small stuffed golden snitch which we hid in the garden and threw a red ball around as the quaffle for a game of Quidditch. I think we also had a couple of the usual party games.