I suggest having a two-part-er.
First, accomplish what you want by having everyone together. Cake, Ice cream, food, and drinks always does well for this. Try to have a single activity that is widely liked, like coloring on the sidewalk, or playing in the water.
Then allow for and encourage separation in the age groups. Play "sandbox trucks" with the younger group while the older group can play something else (sorry 8 is a hard age to come up with a blanket idea).
Almost no activity is going to cover it for both age groups for long. But there are some shared activities that you can start with.
- Water Play
- Arts and Crafts
- Food and Drinks