I'm really not sure that a 12 and 9 yo would care for the aquarium & zoo to be honest. We went when my oldest was 2 and they were great (so was the children's museum). We went again last year and the girls were 16, 10, 9. My friend and her 3 girls (16, 11, 5) were with us. Only the 5 year old liked the Aquarium, the rest of us thought it was really disappointing. The zoo isn't bad, but it's not great either, especially if you have zoos near your home or you go in the summer when the animals hide all day.(Who can blame them?) There is also a splash zone that costs extra. My then 9 year old was the only one who enjoyed the Insectarium. So before you pay the money, think about the personalities of your kids and do a virtual tour online.
Experiencing the crowd and the atmosphere of the original Cafe du Monde is great (cash only), I do recommend it at least once, but there's another one not far down in the Riverwalk "outlet" mall when you want more.
The French marketplace is fun to walk around and look at things, Jackson Square has a lot of artists on the weekends, Royal Street has some great art stores and beautiful architecture. Take the trolley car out to the garden district, Lafayette Cemetery is one of the safer ones to walk around- in the day light, and do stay together. You can buy day passes for the trolley- $3 per person per day.
We had a really enjoyable day at City Park, but it is definitely easier to get to if you have a car.
Mardi Gras World is a fun tour, a bit pricey, but it's neat to see all of the floats in the works, have a bit of sugar drenched King cake and learn about the Krews.
We also did a tour with this woman
http://www.frenchquartourkids.com/ and I thought it was very well done. She had several ages of kids in our group, but she managed well. It was well paced with the sitting and the walking. There was one time I thought she got a bit long winded, but it was good information and the group didn't seem to squirrely, so it was probably just me. I definitely recommend her if you have time.