I think it's a really neat idea. You get 100 points for the coolness factor. But there are a couple of things you might want to consider.
If the party is to be at your home, you might have trouble getting the ladybugs to leave. The last couple of years, we have had a surplus crop of ladybugs in our area and they can be kind of pesky. My Mom lives in a rural area and had quite a problem with them last year and anytime I was at her house I could count 10-12 of them on the ceiling in her living room. She could never figure out exactly how they were getting inside.
Also, as the Mom of a DD who is very afraid of bugs, I think she would freak if one landed on her. We visited the Butterfly Observatory at Niagara Falls last year and she had to leave. She just couldn't stand the idea of one landing on her. You might let the Moms of the party invitees know in advance if you decide to release the ladybugs.
On the positive side, it really would give your DD's party a touch of magic. It's something that all the girls would remember for years to come.
Keep us posted!