How much trouble would I get in if I criticized my girlfriends methods of raising her kids...but come on,,?

Not sure why you're baffled... .it's been common place for decades for parents to feed their kids junk.  Nothing new.

I'd say you need to keep walking that was happening long before you were in the picture.

I don't think it's weird you want to say something, but never at a party.  Also, your title implies your gf has kids.  If so, and this starts getting serious, it would be something you'd talk about when you discuss raising kids.  By then, I assume you'd know a lot more about how she raises them.  Anything is fine at a party.  If they do this all the time, yes it will be a problem.

Get over yourself.  Not only are they not your kids, but sodas and candy at a special event aren't going to kill anyone.  Hopefully she finds someone who has some education and maturity to support her as a partner and a mother.

It was a PARTY. You seem to have forgotten that in your rush to judgment of your girlfriend's family. Likely as not, one of the kids probably had a birthday yesterday, and this was his or her birthday party. Of course the family doesn't eat that way every day, nor do they permit their kids to eat that way. But once in a while is okay. People normally only get one birthday per year, and having soda and candy once or twice in the course of a year has never hurt anyone... even a toddler. COME ON.

You're correct that the sugar-sweetened beverages weren't good for the kids, nor was the HFCS, which is 40 times sweeter than table sugar. But try to remember that this was a one time thing, and not something that was standard procedure.

Have some discussions about what you had as a child, for example - or wht athey had. People don't like to be criticised, so ask questions rather than than tell them your opinion (with which most would agree, no doubt). Try to avoid the word "why" because it comes across as challenging. 

How are we going to know?

What parent puts soda in a child’s baby bottle? That’s crazy! Luckily, they aren’t your kids. You aren’t the one who will have to deal with the sugar rush, and the cranky mood that follows. 

they'll probably be diabetic by the time they get to kindergarten. not your problem. and they'd only get mad at you for pointing out their bad parenting. let it go.