Hi Julie,
Check Revista Planetario for quotes, venues, etc. I use some vendors who are not there, but they do great stuff and the prices are not bad. Where are you? That will also matter regarding price and what you can get for your money.
Peloteros are abundant and all of them do kids parties. From Mundo Discovery kids -expensive, to Isla del Sol, cheap and fun. Most of them include food, cake and sodas for the kids and you pay something extra for the food for the grownups.
If you want something a bit more personal there are also a ton of choices too.
Here are some vendors I have used or had close friends use at one point or another.
For cake, candy bar, iced sugar cookies and cupcakes in any theme I have Ines Farchi at Buenos Dias su Senoria. https://es-es.facebook.com/bd.eventos.infantiles. Not cheap, but not expensive.
For show and entertainment Sybil Pintos https://es-es.facebook.com/bd.eventos.infantiles they do a nice Hi5. I think they are good for a 3 yo.
For babies and for 6-9 years old I like eventos Magoo http://www.magooeventos.com.ar/Magoeventos/Infantiles.html They do a gymboree type of session for little ones that is really nice and for older kids it is wii. Xbox and scalestrix car races. They are very reasonably priced comparing with other vendors.
All these three options are good and if they are not cheap they will not break the bank.
A really nice and fancy coordinator is Proyectos Increibles. They do amazing stuff, I have seen their work on Nordelta and it is jaw dropping, but they are pricy as hell. http://www.proyectosincreybles.com.ar/
Regarding the place. In my case there is a charge for the SUM in our building and does not include a thing. If that is your case, too it is best to hire a kid friendly salon. Most of the places I know are in Belgrano, because that is where my kid's school is.
Babalu http://www.babalueventos.com/#!fiestas-infantiles It is onb te pricy side, does not include a lot of things.
Piccolino Belgrano, http://www.piccolinoeventosbelgrano.blogspot.com.ar/ Not too pricy and nice, a parking lot right on the side of the place. Includes food for the kids, waiters for the kids the gift bag and soda for the kids, you get the rest. Or they can sell it to you.
Domani http://www.domanieventos.com.ar/salon.html The cheapest of the three. Includes the same things as Piccolino, no parking near, but bigger than Piccolino and 300 pesos cheaper. They also have their own show and it very reasonably priced.
The Barbie store in Palermo also hosts birthday parties for girls. It is, as expected on the pricy side.
To give you an idea of the costs. Say you hire Sybil Pintos, she charges 2000 pesos for the show. Includes gift bag, makeup and something else. 2 1/2 hrs. The salon for the place is about 1700 for 2 1/2 hrs. All this for 30 kids. Say you get a candy bar, with cookies and theme related cake 1000. So right there you are at 4700 pesos plus the food and drink for the grownups... If you want more unique things, or a garden party etc, well then it escalates fast.
Cut costs by prepping food for the grownups yourself, La Fabrica bakery has good deals on migas and empanadas, invite a small group of kids, go for party places and peloteros that include more things in the price as they save you time and money. Yes, they are not unique, but they are cheaper. Get sodas, and other supplies in bulk and ahead of time. And the most widely used find a mom in your daughter's class who will like the same idea as you and who will split the cost.
I hope this helps
P
