If you have plenty of time to prepare, you can send out spooky invitations and ask people to come in costume. You can also offer a prize for the best costume. And if there is time you can make some scary food too, like spider cupcakes and witch finger biscuits. If not, most supermarkets have a good selection.
If time is short, you could try some games that take little preparation. A good one is when you buy a lot of toilet rolls, put the children in pairs and get each one to turn their partner into a 'mummy' by wrapping them in the paper. Then they can swap over. You can also try hide and seek in the dark (just turn the lights off, or have just one lighted room - best not to do this with very young children, though, as it can get too scary). Or of course, sardines - most kids love this.
For more ideas, put 'halloween party kids' in a search engine and explore the links you find - there are lots of ideas on line.