It's time to hit the pavement and do the market research.
Start by going to your local video arcade (a very similar target demographic group) and spend a full day there on a weekend and also during the week. Take notes on how many customers come in, and note their approximate ages and what times of the day they come to the arcade.
Next, draw up a questionnaire and ask a sampling of kids (around 50) if you can interview them. Ask them how often they would go to a skate park, what days of the week and what times of day, how much they would pay, and how many friends they would go with. Also ask how many kids would make a park too crowded for them.
Finally, figure out what your capacity is. With this information — customer estimates, frequency of visits and capacity — you should be able to estimate projected attendance for the park.