I don't think that is an apt comparison at all. Nobody is planning/throwing the holiday of Halloween this or any other year. Now if the question was whether it is rude to bring a baby to a party meant for school aged children, that would be a good comparison.
How isn't it? There's an event (trick or treating or a LEGO event). Kids can participate, or not. There are kind of rough guidelines on general ages, but there isn't really any way to enforce them (the Blocktober page says the event is intended for ages 6 and up. If a parent takes a kid to Toys R Us during event hours, which are during regular store hours, and the kid wants to participate, that's not to say they can't. If a parent takes a wee toddler trick or treating, short of shutting the door in their faces, there's no way people can really enforce an 18 month old baby from going door to door.) I'm not saying it's a perfect example, but trick or treating/open event is quite different from a sign-up class/closed event. That was the example I was trying to make.
They are not analogous because there is no set age for trick-or-treating, but there was a set age for the Lego event. Younger children could shop at the store, but the organizer could prevent them from participating. They chose not to in this case, but they could have.
But as it's been said before, the age limit was only posted on the website. It wasn't listed in the flyer in the newspaper, it wasn't enforced by the people working the event, and it wasn't listed anywhere at the event. Yes, they chose not to enforce it, but it also seems like it wasn't widely published in the first place. If the only place someone had heard about this event was from the flyer, they would have no idea there was an age limit.
I think as it happened, OP was fine in doing what she did. She asked the event coordinators if she could gather the supplies for her DD, and they said yes. If they had said no and she had thrown a fit, I'd see that as a problem. If she had seen that it was for a specific age range that her DD wasn't in and had insisted that DD participate anyway, I'd see that as a problem. As it stands, she complied with what the people in charge at that location said, and didn't actively push any children out of the way. And speaking as someone who's been short all her life, if someone reached over me in a crowded area to get things like OP did, I wouldn't have a problem with it. She didn't force her DD into the fray and have her take up space at the table when other children could be there.