We have been raising money for a playground, can we use some of the money already donated to host an event to raise more money for the playground?

Usually. Are you a registered 501c3?

The answer is probably yes. Keep track of every single expense, and all receipts. The donors are your strongest critics. When you spend donated money to make more money, you'd better net a worthwhile amount. They are going to want to know!

Yes, you can use some of the money raised to help pay for the event, assuming the event is specifically about raising money for the play ground. As a ballpark planning figure - it's pretty typical to spend about 10% of the money raised on campaign expenses. The cost for the fund raising event - food, beverages, decorations, flyers, posters, etc. - are valid expenses for a campaign like this.

I do suggest that in your campaign literature you should be up front about how the money raised is being used. For example you might have a list of projects that the money is being raised for with cost estimates, and also include a line item for "fund raising costs."