One huge word of caution. Once your kids travel in an RV, you can never go back, regardless of what the future brings. You are training them at a young age to travel by RV. Once that happens, you are stuck. Watching TV, not stopping for potty breaks, playing video games, eating while rolling. They will not ever be willing to give that up. So realize your first trip will be a major commitment to long term RVing. Your RV is smaller than your house, so everyone needs to adjust to the space, and give room for each other. Hopefully you have figured out your long term sleeping arrangements. Right now while they are young, it doesn't matter, but in a few years, it will.
As everyone says, the black water tank probably has plenty of capacity. We only have 23 gallons for black water. For 4 of us, it has never been a problem. Grey water was 27, which was way small. I replaced it with a 40, which is not bad. For us, we will go about 4 days. Having to break camp once in a single week long trip to empty is not all that bad. I would not bother with a portable tank. Washing dishes uses more water than anything else - plan to use disposable kitchenware.