Some canned tomatoes can be quite acidic. Even if they are sweetened with the cooked vegetables the sauce may still have an acidic taste. Allowing the vegetables to soften for 20 mins, and then letting the sauce simmer for 20 mins, will help with this. If still acidic, try adding a little sugar (or other sweetener). Stir and taste the sauce then repeat until it helps with the acidity. A little salt may also help to balance out the flavours. (If you are serving a baby remove their portion before adding sugar and salt.)