OP, I know a ton of effort goes in to meticulously planning your meals so they come out "on plan". I would have been very bothered by this, too. It doesn't matter that they were kids or adults, I would have been bothered just the same, because someone did something that hurt me and I had to figure a way around it.
Being bothered or upset doesn't translate to actions automatically. For people saying "they are just kids", that does not make the original poster's day any easier to deal with once the meal plan has been thrown off.
Yes, there are lots of actions she can take to prevent it in the future, but I think she really wanted to vent. She is perfectly justified in being upset. It's not like she's browbeating the children... she's just here, expressing her feelings. I don't see an "out of proportion" response to the situation at all.
Stolen food strikes a nerve with me. I have had lunches stolen at work, on days where I did not have a long enough lunch break to go out and buy a replacement, even if it were unhealthy... I just had to go without. As infuriating as it is when a kid does it, when it's a "supposed" adult coworker... I was angry beyond belief!!
I don't put my food in a community fridge anymore. I use a ton of ice-packs and an insulated, soft-side bag to keep it cool on my desk until lunchtime. Since it sounds like food theft and eating of off-limits foods is not uncommon in that household, you may consider this. Bring extra ice-packs to freeze in your friend's freezer overnight, then change out the ice-packs in your bag before you go. No risk of them eating those blue-goo Icepacks, I hope...