A husband and wife should not go off on a pleasure trip together when the wife is
niddah. What you are describing seems much more like moving your entire family from one location to another. This sounds appropriate when you are
niddah.
The issues with mixed swimming are seeing people underdressed and being underdressed. It is less of a problem for women to see men in bathing suits than the reverse. Therefore, for you to be at the beach dressed is not a problem. Similarly, for your 4 year old boy to see women in bathing suits is not much of a problem. The question remains with your girls. You do not mention their ages. If they are older (say above about age 6), they should also be properly covered. Furthermore, there is the issue of chinuch in term of their understanding the problems of mixed swimming.