From what I understand, you can use the promotion (BOGO free) as many times as needed, but it has to be done with separate transactions. So if you have 2 adults and 3 kids, you could so 2 transactions - 1 adult/1 kid on one and 1 adult/2 kids on the other, but one would be full price.
There are some single $10 off coupons out there too if you wanted to do the 5th person separate at a discount.
Also, wanted to add that I just used the BOGO code yesterday and the price per adult was $65 plus tax, but when I got my email confirmation, it said my credit card would be charged $55 plus tax, which is what the price was originally when the deal came out.

So we'll see...