Birthday Parties birthdaycontract All parties are $375 for 1-1/2 hrs, you may add an extra ½ hr for $75. Party base prices include 15 children, $12 each additional child. Parties include one hour in the gym & ½ hr in the party area. Flip-Flop Gymnastics supplies the balloons, paper goods, invitations, & a t-shirt for the birthday child. Parent only supplies whatever food & beverages you are serving. Birthday parties are held on Saturdays or Sundays. Choose from 1:30-3:00pm or 4:00-5:30pm on Sat., times for Sun. are flexible.