Skipped over all the drama speak, but here's my take after reading the first post and a few of the follow ups.
She's under 21. You're probably not much older (since you're mixing booze into a slurpee lol
There's a 95% chance you're not going to marry this girl. You're two young kids that want to have a totally awesome time.
IMHO, your plan sounds great.
When you get to the park, ask her if she'd like to spike it. Maybe offer to only spike one of the drinks. It's all about what SHE wants.
Go have fun.
And remember to use a condom.