i can totally relate, lynn. i too was naive as an early adult. what i learned in therapy is to set boundaries with others and if they step over that line, i remove myself from the relationship somewhat. in other cases i remove myself totally from that person/persons. in a general sense what you stated is more true today than ever before. i believe many ppl have corrupt values and i need to be alert to that. not paranoid, just very aware. the flip side is that i try to choose my friends wisely. i know that they can be trusted completely. but this took practice to learn how to choose wisely.
as for family i have learned i cannot trust them at all unfortunately so i keep a healthy distance to protect myself.
re what you tell your children...i once heard a saying-if you want to keep a secret don't tell anyone. makes sense cause it's a secret. it is too tempting to most ppl not to pass on their interpretation of what i say/secret.
and to answer your question, re can one "assume" ppl can be trusted? my answer would be no. but what we can do is be optimistic but careful and choose wisely by ppl's behaviors.