Don't be afraid to ask for help. If this is an adult party, be sure to text your people who are coming and tell them what they need to bring. Don't leave it up to them to decide what to get, it is best you tell them. Like, Sally bring the soft drinks and Sam needs to bring napkins. For a kids birthday party, your family would probably happily oblige. Contact them and see if they would be willing to help out.
Registry isn't a dirty word but some people find that it is. When I was coming up with my bridal/baby showers, I had people simply refuse to go off the registry. It's like they didn't want to be told what to do. They wanted to rebel against the registry. So, don't be like them and pick up the registry at the front of the store before you get their gift or find one online and get something off of there or a gift card to that store works just fine. Then they can get whatever they want!