Well spotted JW. I suspect that the camera was at TR 34153 70572 (or 51°23'09"N,001°21'50"E if you prefer). It must have been low tide, and the family were fairly well out, slightly to the east of the end of Westcliff Road. On the skyline to the right of the photo you can see the house with the distinctive turret, this is number 1 Royal Esplanade, on the corner with Westbrook Gardens. The slipway at the foot of Westbrook gardens is behind the family, as is the ramp leading up from Westbrook Promenade. The latter looks like it leads directly to the Royal Hospital, but is actually at some distance, perspective is playing tricks here.
For those of us relying on maps, it is only fair to point out that this is Westbrook rather than Margate itself.