Q1: I saw Lee Ann posted that she got there early last time, so how early was that?
From what I've been reading in other threads, the port opens at 10.
Q2: Our rooms are 7516 and 7518, category 5 I guess. I saw room pictures and some of them have bunker beds and some only with one sofa bed? We are planning to have one of the boys staying with grandma and grandpa, but just in case they both want to be with mommy and daddy, we want to be able to do that. Also, is the main bed queen size?
I think bunk beds and queen combos are the default for Cat 5. Just note that your stateroom attendant will have to unlock and pull down the bunks so let him/her know the sleeping arrangements before you're ready to crash for the night.
Q3: Chrisann asked me if I have signed up for the kids club yet. I obviously have not!! I will do it tomorrow!! So, if we drop them off at the kids club, do they stay there all day? what if I want to see how they are playing, take pictures, etc., can we check in and out freely?
You can sign them up for the clubs online. You'll receive a pager, so if they want/need to be checked out, you'll be alerted and can come get them. Reading other people's trip reports, you can treat the club like a revolving door and no one will say a word.
Q4: I am disappointed to know that the water temperature will be in low 70s.

Any recommendations for other activities? My 4-year old is not old enough for the Dolpin thing, my dh and I have done it before we had kids, so I am thinking about only just doing the stinray? Anyone done the bike rides?
No experience here, sorry.
Q5: I am still confused on the dinning. Sounds like the adult only dinners are fully booked. Anyone doing the lunch? We are booked for second seating, so if we are hungry, can we eat a small meal first at the buffet?
Palo, the adults-only restaurant, is only available for dinner on our cruise. You can always grab a snack from deck 9. One tip I've read about on other threads is to order a cheese and cracker plate from room service to eat while you're getting ready for the evening shows. Sounds like great advice!
Q6: And I really want to confirm that there is NO formal night on our cruise, because bringing suits and dress shoes for dh and grandpa will be a real burden. I will bring a dress anyway, but if we don't need the suits and dress shoes for men, that will make a HUGE difference. Also, I read the dress code thread, they say "dress up" with "jackets", what exactly are people waring? Polo shirt OK? Or they do need a sports suit or something?
No real "formal" night. You'd want to dress up if you do manage to get a Palo seating, but polo shirts are fine for the main dining rooms.