I bring my acoustic guitar and play a full set of kid songs for the birthday child, all the friends and family. I usually sing and play for approximately 30-35 minutes playing all those songs you've known and loved for all these years (and more) mixing the standards with some new songs as well. I also bring a suitcase full of egg shakers, bells, scarves and a ton of animal puppets to make it a great interactive experience for the kids (and even some adults!).
After that, operating on the assumption that the adults are very tired of hearing those same kid songs for the millionth time (that day?), I would love to play a whole set of songs for you and your friends (for at least as long as I've just played for the kids) - sort of like a live jukebox. I know how often parents are going to birthday parties (like every weekend, right?) and I like to do what I can to make their experience fun too, you know? I also play in a 1970's cover band and know lots and lots of songs from that time period (as well as many other periods as well). The bottom line is that I look to show everyone, both kids and adults, a really good time.