There is not a lot of choice at low tide.
I would go for a picnic at East Head, if I were you.
Also, depending on the age of the family, you could try going up Emsworth channel to see if you can see any seals.
Sunday looks a lot better for a spin outside.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams