It was a scorcher last Friday afternoon (and pretty much every day afterward), but little Sara Lakin of the South End and her caregiver, Joan Menzie Brown, had the right way to cool off – at the season’s opening of the Frog Pond on Boston Common. Hundreds of children flocked to the Frog Pond to escape the heat and enjoy the fun with the Boston Parks Department. Commissioner Chris Cook gave the countdown,and then the kids dashed into summer.