It's actually surprisingly simple to throw a great party that all ages will enjoy. Here's a few tips to make it great...
Choose somewhere big enough. Make sure there's plenty of room for adults and kiddos to spread out.
Themes make it easier. If you have a theme (be it holiday, season, event, topic, etc) it's so much simpler to plan. Keeping it non-age and non-gender specific (such as tropical or beach in summer or snow or igloo in winter) is the key.
Keep it simple. Don't be extravagant with food or decorations just pick a theme and create the environment around that. With a varied crowd it's about atmosphere.
Finger food is your friend. Children adore it and adults think its cute. Just have plenty on have and feel free to be creative. Kabobs are a fun option as is using cookie cutters to make food into shapes matching your theme. Simple creative containers to serve it in with creative food labels is also a fun addition.
Have activities available. A table with crafts and/or coloring pages for the kids to do and ways to keep children busy and active will ensure less frustrations arise. If you plan games leave it open for who can participate and don't plan anything that is mandatory for people to participate in.
Have fun. Plan it, prepare it and enjoy it once guests arrive. Try not to put yourself in a position where you're having to prepare things or do a lot of running around once the party starts.