It is so normal to have crushes like you said. They are only 5 and it is also normal to pick that one friend that you hang out with all the time. The good new is, these things are short lived, crushes go in and out. Also, the chances of them being in the same class next year is probably pretty slim as well so it will fade when they cannot be with each other every day. You may be getting worked up a bit too much.
Another way to look at it, if this boy was a girl they would write notes, only want to hang with each other, draw pictures for each other, etc. and no one would worry. Given that they are boy and girl, you just need to clarify that when you have a boy as a friend, you do not go to each others rooms together, and you do not kiss, etc. She will understand that there is a difference, tell her she is too young to have a boyfriend and just keep the wedding and boyfriends until she is older.
That being said, at 5 you have all the control and you do not need to make a federal case out of it. If you are not comfortable with her going to the party, then make plans to do something else and let her friend and his family know you have plans and cannot make it. Drop off a gift, no big deal. If she were to go to the party, then just stay there with her and monitor what is going on. It may be a pool party but I highly doubt they will be swimming unless they have an indoor pool. I am thinking it is pretty odd.
In short, just watch her, don't freak out. Kids don't identify things at this age like we do. She just loves her new friend. He is a boy so she made the natural connection to marry him when she is older. That tells me that you have a strong home life and she wants that one day with her little friend. My son at 3 is convinced that he is going to marry my best friend and follows her around constantly. They are innocent I can assure you. Just stick by her and make sure nothing funny goes on.
Some of the pro-active things you can do going forward:
- Have their teacher make sure they do not sit by each other during class
- Stay at the party if you go and offer to help out that way you can keep an eye on her and how the family is.
- Do not let them hang out in their bedrooms or worse shut the door
- Encourage good friends and nothing more.
They will be fine! Good luck!