There are two distinct views of will power in psychology and behavioral econ. One is that will power is a fund, which gets depleted the more often you draw from it. The other is that will power is a muscle, which becomes stronger the more often it's exercised.
The will-power-as-fund viewpoint tends to support paternalistic policies, because the modern capitalist system presents people with numerous temptations they have to resist. A person faced with too many temptations will deplete his fund and eventually give in to temptation. The will-power-as-muscle viewpoint resists paternalism, because shielding people from temptations will slowly weaken their will power.
So which viewpoint is true? I would say the muscle viewpoint includes the fund viewpoint. While it's true that in the long run a muscle gets stronger as you exercise it, in the short run a muscle can indeed become exhausted, just like a fund. Exercising your muscles regularly and strenuously is a way of increasing the amount of strength you can deploy at any given time. Put more simply, exercising your muscles increases the size of your strength funds. And that, I think, is how will power works as well. You exercise your will power by resisting temptations, first smaller and less frequent ones, then larger and more frequent ones.
Of course, it's possible to exercise too much and actually do damage to your muscles. This is especially true in the early stages of an exercise regime. In terms of strengthening your will, it's better to successfully resist smaller temptations than to give in to larger ones. Hence our greater willingness to impose paternalism on children (paternalistic does mean "like a parent," after all). Children are unlikely to have developed strong will-power muscles yet, making them especially vulnerable to large temptations and unlikely to be strengthened by them. Adults, on the other hand, are presumed to have developed the necessary muscles, or to realize they need to.
Accepting the will-power-as-muscle viewpoint draws attention to the unintended spillover effects of nanny-state policies. Policies that shield people from temptation X may actually reduce their long-run ability to resist other temptations, including temptation Y. This creates support for policies that shield people from temptation Y, further enervating the will power required to resist yet other temptations. In addition, the knowledge that the state will protect people from certain temptations reduces their incentive to strengthen their will power, making them even more vulnerable to future temptations. The result is a slippery-slope process in which paternalist policies create the very conditions used to justify them.
(I don't recall where in the literature I encountered the "fund v. muscle" distinction, but I'm pretty sure I didn't make it up myself. If anyone knows the source, I'd be most grateful if you gave me the citation.)