Oh, what fun! Running is fun in general but running for a charity event is always a great time. It feels good to know you are running to support a worthy cause, and most of the time if a charity is involved, you are more likely to be able to convince your friends who might not otherwise be interested in running, to sign up for the race with you.
However, running spikes are probably not exactly what you want for an event like this - at least not track running spikes that are intended for short sprint distance events. You are smart for thinking about the rocky or uneven terrain, and trying to prepare yourself for it to avoid injury and stay safe on your feet, but track spikes are made using a lot of little spikes to grip the track, but these spikes are not as sturdy. If you DO really want to get spikes, you can get distance spikes, intended for longer distances on the track. If you buy ones that are thicker and sturdier (i.e. they need to be wider and longer than track spikes), these can be worn for cross country events. However, the downside of these spiked shoes are that, because they were made with the track in mind, they probably will not be as sturdy and durable of a shoe overall than if you were to purchase other running shoes. Track spikes, even distance spikes, have very little padding, support, and cushioning (though they are quite flexible and lightweight). While you might enjoy the quick turnover they give you, your feet and ankles might start to ache near the middle or end of your run simply due to the lack of support - and this is especially true if you are not used to running in them. It is also important to note that if you intend to ever participate in any serious cross country events or races, most prohibit metal spikes.
So if your heart is set on a pair of shoes with spikes, then go only for the distance spikes. But you are just as well off buying traditional running shoes that are thicker and offer more support. Alternatively, if the uneven terrain is really of concern to you, you can also look into a good pair of trail shoes. These shoes come with bottoms made to grip different terrains, but are thicker, heavier, and more durable than spikes.