With less then 20 people coming you should have no problem feeding everyone for under $100.
We have at least 40 people at DD's bday parties every year and I am always able to feed everyone for less then $100. Things we've had in the past were:
1st bday (I probably spent just a bit more then $100) - baked ziti, fried chicken (I bought it from Meijer), tossed salad, rolls, pistacchio salad, lemonaide, punch and bottled water, cake and ice cream
2nd bday - Hot dogs, chili sauce, mac & cheese, chips, lemonaide, iced tea, bottled water, cupcakes
3rd bday - I wasn't working so we just did cupcakes and had chips and pretzels out, lemonaide, Capri Suns
4th bday - Sloppy joes, pasta salad, chips, and I think some type of other salad, maybe cole slaw??, lemonaide, bottled water, Capri Suns, cake & cupcakes
This year we are debating on what to do. DH wants to do fried chicken again and order a chicken strip tray for the kids. DD said she wanted pizza but with as many people as we have coming I can't see a budget way to do it. I suggested a taco bar and taco salad. At this point the pizza is out and DD seems fine with that but I need to get prices on chicken and roughly estimate how much a taco bar would cost us.
IMO, if I'm feeding people, they'll eat what I feed them

I don't put too much thought into it. And I certainly don't go overboard with the amount of food I make. We have our parties at lunch time so I figure its a lighter meal and therefore they don't 8 choices for food. 3 or 4 is sufficient.
I hope your DD has a fantastic 1st birthday!