They'll be fine.
When I asked my (nearly 6 year old) niece to be my flower girl it was just after the Royal Wedding.
The conversation went (on the phone, on her walk to school with her mum*):
Me: DN, and I are getting married. Would you like to be our flower girl?
DN: Will I wear a princess dress?
Me: Yes.
DN: Can I wear fairy wings?
Me: Yes, if you want to [for I am an excellent aunty - she didn't in the end, though]
DN: Will I get a day off school for it?
Me:No, it's on a Saturday.
DN: Will it be on TV? With a kiss on a balcony?
Me: No. But there will be photographs.
DN: But why won't it be on TV? You should be on TV.
Me: I know. But it won't. There'll be cake and pop and music to dance to and will be there.
DN: Oh, OK then. But only if I can have a beautiful princess dress.
Me: DN you will absolutely have a beautiful princess dress.
*I had previously checked with her mum that it would be OK, so she wasn't totally blindsided by this.
When I was little, I asked my DUncle if he was getting married (there was a spate of weddings in the family). He said no. I asked if he had a girlfriend. He said no, and then "talk to your mum". I did. She said that DUncle didn't like girls, he liked boys, and that that was OK (this was the early 80s).
I went back to see DUncle the next week and said "I know you don't like kissing girls, you like kissing boys. Do you have a boyfriend. Is your boyfriend?" and that was it. All over and done with, and I got a Rubiks Ball for being "cool".