Ok, this is what we did. The party was for 10 dc (my 2 plus 8 guests).
We decorated by making signs painted on cardboard for the different rooms of the house - the front door had a Hogwarts sign, the living room was the charms classroom, the dining room was the great hall, the kid's bedroom was the Gryffindor dormitory, the spare room was the positions classroom. I also stuck a print out of moaning myrtle on the inside of the loo seat, wrote "the chamber of secrets has been opened" on a mirror in lipstick and got out any toys the dc had that were vaguely harry potter like (such as cuddly owls and dragons) and popped then around the place. We had made Quidditch hoops by taping hula hoops to some 2x2 wood which we stood up by hammering a garden stake into the ground and then taping the 2x2 wood to it (used gorilla tape which was very strong). We used the green opposite our house for this but you might have a garden? The final prep was hiding a bunch of plastic spiders and rats left over from a Halloween party in the garden, as well as a giant spider. Do and I also dressed up in wizards hats and black clothes and told everyone we were professor mcgonigal and professor snape.
The activities were:
Lesson 1 - charms. We gave out wands (i got a pavk of 11 wands from ali express for £10, these acted as party bags too). We started with teaching a sleeping charm, we demonstrated it on an adult (so the dc got the idea that they had to dramatically fall asleep) then let them practice in pairs. Then we split the group in half and I did wingardium leviosa using a wand with a small magnet glued on and a feather with a small magnet glued on. Dp did incendio as w happened to have a remote flint flasher that meant he could make an object flash when the spell was cast.
Second lesson was care of magical creatures. Sent the dc out into the garden to find the hidden creatures, and told them to beware of Aragon the spider. They predictably chased each other with the spider a bit.
Third lesson was potions. We had set up a camping table with lots of ingredients in small potion bottles bought from eBay and we had some plastic cauldron left over from Halloween. I got the potion ideas from this website.
Fourth lesson was Quidditch. We played 5 a side with 2 chasers (who tried to put the waffle through the hoops), 2 beaters (who used a bluster to tag the chasers and freeze them for 10 seconds) and a seeker (who would end the game by catching the tiny bouncy ball that me and dp threw to each other across the pitch). We swapped around so everyone got a go at being seeker.
After that we did "the great feast". It was mainly hotdogs and pizza, I'd made owl cupcakes and some golden snitch sweets by gluing wings to ferraro rocher.
After they had eaten I broke the news that the philosophers stone had been stolen, and the thief had only left a torn up message. The dc found all the bits and put them together to find a prophesy that the mirror of erised would show how the find the stone if they sprayed it with water. I had bluetacked the words from the mirror of erised around a mirror in the spare room and I had used rain-x to carefully paint words on the mirror that were invisible when dry but showed up when sprayed with water. The dc did that and were told to go to the charms classroom. Meanwhile dp had put on a mask and was pretending to be a cave troll. The dc had to cast the sleeping spell (do did dramatic acting) to get past him to rescue a big plastic gem.
After that the dc did some free play for a while while we cleared up, then we put on the harry potter film before bedtime.
We had one last activity where a couple of cloaks were delivered outside the bedroom door with a knock and a note saying they were invisibility cloaks to get the midnight feast the house elves had left in the kitchen. Dp and I patrolled as teachers and sent the students back to their dormitory if we saw them not in a cloak or they made too much noise, we had left a basket of snacks and drinks for them to sneak by and get.
And that as pretty much it! Sorry it is a bit long to write down.