I need some help with my anti-paternalism defense.

I'm a long time libertarian considering the leap to become a full fledged ancap. Actually, I may be mostly through that leap already. The problem I will attempt to describe here, I think, applies to either realm.

I participated in a discussion recently here in r/an_cap wherein I questioned how to logically differentiate between children (and the mentally deficient) and "full" people. There were some helpful responses suggesting that we just try to handle it fairly, but I was left with more questions than answers.

If we concede that children should be protected from the full responsibilities of their uninformed/poorly considered decisions, how do we reconcile that with allowing full freedom of contract (and the corresponding full responsibilities) to imperfect adults? To clearly illustrate, I (and I suspect most people) would find it immoral to hold a 2 year old to a contract exchanging lifelong servitude for a sugar cookie. Most of us would find it acceptable to use force to restrain that 2 year old from running into the middle of a busy highway. The reasoning goes something like: "the 2 year old does not have the experience, the understanding of life, or the mental capacity to fairly evaluate such a choice."

However, even though I like to consider myself to be of above average intelligence and prudence, I make decisions without properly evaluating their consequences more frequently than I would like to admit. I have always felt that those decisions and their consequences were my own responsibility, but have also felt that there is something unethical about people who intentionally prey on the situationally predictable poor decision making of others. (I still want to find a way to avoid justifying initiating force against them.)

So here's the conundrum: if we concede that someone (parents, other family members, or just "the community") should use force, if necessary, to prevent children (and/or the mentally ill or disabled) from acting against their own interests, how can we draw a clean distinction between that and the paternalistic state, who claims, after all, to be simply acting in the "best interests" of us (frequently genuinely) dumb, ignorant, cognitive-bias-prone schmucks? If we can't draw a clean distinction, do we really have a logically sound argument against arbitrarily intrusive state actions?

I apologize if I've done this wrong or committed grammatical atrocities; this is my first ever "link" post, and I'm doing it from my phone.

EDIT: I did screw it up and posted an unfinished draft. Edited for completeness.