instant food on shabbat

Question

may one add very hot water (yad soledet bo) to instant thing on shabbat such as soup mix, hot chocolate, oatmeal, etc. I would guess that these are all cooked and processed already. Is there a specific way you must add the water? b) since milk is pasturized can you put it on the hot plate and take it off before it gets to yad soledet bo. c)can you use a thermometer on shabbat to tell you if the water is yad soledet d)can you use a thermometer to check a child's temperature?