1) There might be a hundred and fifty people or more.
2) They will not all be people you invited, at least not by name. They will be parents, grandparents, aunts, cousins, and maybe neighbors of the people you did invite. This is totally expected. It isn't bad manners to bring uninvited guests; on the contrary, it's bad manners to be so stingy that you don't provide enough refreshment for all the people who might show up.
3) Therefore, as you might expect, they cost a mint. A party held up the street from us (for a one year old!) involved a closed street; rented tents big enough for a mid-sized circus; clowns; and a catered meal. The goodie bags they gave out - to approximately 75 children! - were impressive.
4) Mezcal is provided. Very occasionally, I have seen beer and wine provided for the adults at a children's birthday in the states, but I think it's fair to say that the consensus opinion is that alcohol has no place in the same room with balloons or anywhere in the vicinity of a bouncy house. Here, a good host provides a bottle of mezcal at every table, as well as beer and soft drinks. No-one gets snookered at a children's party (it's rude) but no-one is expected to attend a party of any kind without a little booze.
5) If you don't let your kid eat sugar, or if s/he has some sort of food issue, it's all on you. I've never seen so much sugar in one place in all my life - cakes, coca-cola, jello, showers of candy from the piñata, ice cream, more candy, more cokes..... As far as I can tell, no-one restricts their children's diets in any way at all. And their behavior precious little.
6) Although children's parties don't get rowdy, there will be at least one incident of shocking gang violence, and somebody will get pummeled with a stick.
7) They might even lose their head entirely.