Bet you are wondering how on earth we got that circle so perfect (admittedly it was more perfect before pesky children climbed in. I hate it when real life gets in the way).
Actually there is no ‘we’ here. It was all David.
He had 3 pairs of hands, two broom handles, one long piece of rope and several shovels.
Find your center spot and plant the broom handle. Tie one end of the rope to the planted broom handle, tie other end to second broom handle. Walk around in a wide circle tracing in the sand as you go. Repeat for wider circle. Dig.