I have also had this problem. I hate the thought of my oldest 6yr old DS soon to be 7 in dec, going to the public toilets by himslef. There was a story when we first moved to WA about a young 7yr old girl who was rapped and killed in a public toliet in the mall while her dad and brother waited outside the womens toilets !!!.
I just done feel comfortable with it at all. My DS is a very big boy and people often mistake him for being older than he is, which is hard in this situation.
There was a poster put up at our public toliets in the mall saying that no children older than 6 should be in ther oposite sex toilet. I dont pay any attention.
Also I think it is a bit different for boys to go into the girls toilet as ours are all in cubicals and nothing will be seen, where as the mens have urinals.
I was no help sorry!! I think like cookie monster said as my son is the oldest its hard but he will be able to take my DS2 when he is old enough, IYKWIM??