I would think a chain link fence on all sides with a locking gate, several feet from the hives, and also a chain link roof might work to keep people and their things out. It won't keep kids off, and keep them from getting stung, though. I've seen people with "hive houses" that are well ventilated and provide shelter for their hives. This could be the best idea, since it could be locked, things can't be thrown at the hives, and it could be decorative.
Lights and an alarm system won't help much, but a closed circuit video camera could.
I think the warning signs are likely to generate the wrong kind of attention. First, the people who are afraid of bees will use it as "evidence" that they're dangerous and shouldn't be in a public place. Second, warning signs are like a vandal magnet..."oooh, what's that? Must be interesting if they want me to stay away!" You'd be better off with informative signs...teaching the basics about honeybees and (perhaps) visually showing the difference between them and wasps, yellowjackets, etc.
You'll also need to think about a water source if there are water fountains in the park.
I think going with a lesser-trafficked park might be a better deal, too.