edited: I assumed you meant a party at a movie theater?? If it was in their home I might have let my kid go after having met them, but not in a public place. but I've also never heard of a home movie party.
and it's less about people hurting or abducting them at the theater but them getting lost or scared of the movie/dark or things like that, I wouldn't put my baby in that situation at that age.
My ds was invited to a movie bday in first grade. Also did not know the family but had actually heard not nice reports from my ds about this kids behavior. He wanted to go, so I agreed, but we went as well, and I am soooo glad I did. holy cow, not ready for my 6 yo to be running around the arcade before the movie, with a bunch of other customers there, not ok with the whole concept of a group of young ones at the movie, with just the mom and grandpa to run them to the bathroom etc. Grandpa was a little shady looking himself, and of course I never think to ask the mom who she plans on having help her. PLus at 4 my kids hadn't been to many movies because the volume is so loud and not much is suitable for that age. and at 4 I never dropped off at parties so short answer, NO way would I have let my kid go.
but yes, I might have been offened by your wording, had you said ds didn't want to go with out you, or that grandma's planned changed and she wants them for the whole weekend and has plans so can't bring him, you know something like that, it might have taken away the sting. and while it's great that you asked to meet them in the first place if I were them I might have found it a bit forward, but i'm weird that way.
having said that , my weird advice would be to stop by with the gift after school, so that they can see you still want to be friends. I wouldn't call in advance just show up. otherwise they might keep putting you off, and by sending it to school you don't get a second chance. take the opportunity now, while you have a purpose like dropping off the gift. and even if they are like me and don't like unexpected company they can't be mad if you bring their kid a gift!!