Both families should weigh the harm to others from their actions versus the harm to themselves from changing their actions.

Not eating a peanut butter and jelly sandwich isn't harmful; one can survive without eating one's favorite food in school every day. Peanut allergies on the other hand, can be fatal. Someone ought to remind the principal (a) that children are not compelled by law to attend shopping malls, as they are school, and that (b) shopping malls aren't vested with the same obligation toward student welfare, that schools are.

Alice's parents should also weigh the harm to others versus the harm to themselves. In this case however, the decision is subject to federal law, and the law at issue would probably not satisfy federal constitutional guidelines. Alice's parents are stupid not to vaccinate, but they could consider a constitutional challenge if they have the resources to do so. The extent to which they succeed might be different if Alice was unvaccinated for another reason - religion is constitutionally protected. Furthermore, if Alice is not medically eligible to be vaccinated as some children are not, then there might be a claim that the blanket law is unduly restrictive to rights of access to public spaces.