I have never seen those in Canada. We have these Heinz "mashed potato" meals....but I can never find them. Apparently kids just love them but they are more for toddlers.
If you open a jar, take out what you need and you can refridgerate up to three days. If you start out with 1/3 of the jar, it should last three feedings. Pretty soon they will eat half - all depends on what it is. I found out that my son loved broccoli/chicken/cheese casserole more than pasta/meat sauce. Then before you know it, he will be eating a full jar.
It may take a few tries.....but soon you will know how much to feed.
Good Luck!!
Stacey