Question please about unvaccinated child

[lvchefmel]

Hi, I am struggling with a question and would love some advice. I
found a family that is interested in caring for my six month old
daughter who has had no vaccines. It would likely only be for maybe
3 months while I do my internship and maybe some extra babysitting
on occasion. My concern is that they are world travelers. They do
mission work, though an amazing thing to do, puts them in contact
with possible diseases from these countries. They recently were in
Indonesia and their daughter is leaving for the Dominican Republic
soon. I really like this family and the wonderful things they are
doing for others. I just wonder what the possibility of them
bringing something back that could affect her or her two brothers.
I also have a 2 1/2 year old son, no shots and a 5 year old who
stopped getting them when he became autistic. Am I over-reacting in
my concern? Thank you so much for any advice.