If I'm very short on time, I'll often use canned or frozen veggies—check the label to make sure there is no sugar or other additives. Pre-chopped veggies are more expensive but great for purees (I prefer fresh for crudités). Use them as soon as possible after purchase…the shelf life of cut-up vegetables is significantly shorter than for whole veggies.
Purees can just as well be added to store-bought foods such as macaroni and cheese and boxed cake mixes. I'd recommend adding the puree incrementally and tasting after each addition, so that you can judge the correct balance of flavor for your family.
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