I wouldn't go smaller than a 13 if there are 5 of you, especially with those ages of kids. Look to something with 3 thwarts and some space. Most 12's are much narrower and if you all average out to 100 lbs each, that is 500 lbs of people. Then ad some lunch and gear, and you are looking at 550-600 lbs. What if you kids want to bring a friend? Are you going to have space? Smaller rafts with a smaller footprint can handle the weight, but the raft will be drafting more water. If this is your summer family boat, that means low water in most cases. A larger boat will draft less water.
One thing I like about my Sotar (there are things I don't like, I'm not in love with my boat) is that it is so damn light, and it drafts hardly any water when we paddle. My wife and I can paddle it with some "help" from the kids with no issues. My Dad has a Riken Pioneer that is a 12'8" long self bailer, and it's a great size paddle raft. It is the same width as a 13' raft, just a few inches shorter. It is getting hard to find used Rikens, they have been out of production for over ten years now. His boat was made in 1996, and is still in fantastic shape.
Wishing I was on the river instead of surfing the web...