Can an adult epipen (epinephrine) be used on a child during life threatening anaphylaxis? My 7 &9 yo sons have had hive reactions in the past. I keep an epi for me.

No, need kid's dose. First off, if your kids have only reacted with hives, you need to not over-react unless they have a history of anaphylactic reactions to various things. This should be investigate by a qualified allergist if you are concerned. Second, if their allergies warrent using an epipen, (epinephrine) then need to have their own, which will contain an kid's dose vs an adult dose.
Yes. The dose of Epinephrine is 0.005 ml per pound. So a 30 pound child (2 years old) needs Epipen (epinephrine) jr. (0.15 ml). At 60# we switch to Epipen (epinephrine) (0.3 ml). Many allergists consider the junior dose inadequate for children over 50 lbs. I switch my patients to Epipen (epinephrine) when they reach 54 lbs. If your kids were my patients i'd tell you to go for it but you should discuss this question with their doctors.