Well, my Passporter calls the excursion "Tulum Ruins and Beach Tour" but I just checked the
DCL website and it is only called "Tulum Ruins". On the DCL site in the description it says:
"
A small beach is located at Tullum. Guests must be able to climb stairs to enter and exit the water."
Now I am not sure about the beach. Would you have been able to go on the beach for a while if the weather had been nicer? Did you have any time that you were on your own or were you with a guide the entire time (at the ruins)?
The pics that someone posted of the beach IS the beach that we walked down to and we did stick our feet in the water. There are NO stairs to that little beach - just a slope is what I remember. There are not changing facilities so you'd just have to wear your suit under the clothes and dry off before getting back on the bus. Like I said it was raining (fairly hard rain) during our whole trip so the beach wasn't popular that day! There were a few people who may have been locals in the water.
There were stairs to somewhere (another beach area?) but they were roped off. I heard they were damaged in a recent hurricane but I don't know why they were roped off exactly.
The DCL excursion DOES include your ticket to ride the small tram (er....tractor pulled thingee) from the parking lot to the ruins - you do not have to walk nor pay to ride that tram - it is included or was in our trip last month.
We were with the guide for about 45 mins at Tulum (where the ruins are) then I'd say we had another hour or two there on our own. It is definitely an hour bus there an back I think an adventurous 6 year old could handle it but they might not be fascinated - LOL!
I can't compare Tulum to other sites but people say it is small. The temples are not as grand, etc. I wouldn't discourage anyone from visiting an archaeological site and learning more about the culture and history, etc. I'm glad I did it and if I did the Western often I'd probably go back but I'd like to see Xcaret too. Tulum did make me want to see Chichen Itza and other more extensive sites. My son wants to do a social studies fair paper on Tulum.
I'd say go if the adults and 2 older kids are wanting to see it - there are enough of you to keep the 6 year old company unless she is difficult in these situations. You also have the option of leaving her in the kids club on the ship all day - they'll take her to lunch, etc. I know some people aren't comfortable with that though - I certainly would book the excursion through DCL if you did it. I also don't think you could go wrong with Xcaret.
I just want to add that I don't know where that dolphin swim is that folks do at Cozumel but I talked to a few who wished they had booked the dolphin encounter part directly rather than through DCL cause with DCL you just petted the dolphins but booking directly you got to swim with them.
And I'll add BUY
TRIP INSURANCE! I little toddler on our cruise had a freak accident in Cozumel - just slipped and fell and broke his leg so badly he had to be medically airlifted back to the USA (to the tune of $30,000+) (he is o.k. but had to be in some sort of body cast for many weeks). They were Disboard members too and they had insurance - cause we're all too smart to cruise without it right?
). I am not too concerned with having to walk around since they are all used to this at WDW. 
