Do you want to be able take all 3 kids
out at the same time? Are you planning to take them out on the water
yourself? or Just let them take the boat and go play?
Sevylor makes some pretty decent inflatable
kayaks. something simple like this
A dinghy type of boat would be best for trying to fit everybody into at the same time. The inflatable kayaks are pretty stable and I wouldn't worry about capsizing them, but a dinghy type of boat will be the most stable and can handle kids moving around much better.
A decent inflatable boat would be something like an Intex Seahawk.
I wouldn't worry about getting something that will last forever. Both the Sevylors and Intex boats have a good reputation and should be more than good enough for a few outings each year, for several years.