FWIW, a "school party" is a fun treat, but it does NOT count as a birthday party.
I'd recommend separate parties. Maybe legoland with 2-3 friends + sleepover. How old is DD? For young kids, a good rule of thumb is no more guests than the child's age.
BTW, I spent $10/kid for matching outfits at DD4's b'day. The kids LOVED them, I mean really really loved them.
