On your invitation, try wording like "Activities for the kids from 2 - 4 p.m." as a footnote to the time.

-OR-

Create different invitations, one set saying 1-3, another 2-4, and so on. Though, I think that could backfire and cause problems.

I have been to parties like that.....
It works well to stagger the crowd, ask those with kids early and plan food for them, and ask those without kids later......and plan adult food
Linda C

For the wording on the invitation..perhaps state:
"Families Open House...from 2-4pm"
"Adults Only Open House..from 7-10pm (or whatever)

Gear your food selection accordingly. You might want to go with a variety of lovely appetizers for the adult party.

Linda and Sharon have a great idea...and Sharon's wording is perfect. That can't possibly offend anyone as the options are open!

Have a great party and Happy B-Day!