At 30 months I am not sure this is any worse than putting it on their thumbs to stop them sucking them - I have a thumb sucker who is reducing her thumb sucking to only at night time through bribery at the moment but I wouldn't rule out the nail stuff (because the thumb sucking is such a reflex thing she doesn't even know she is doing it half the time). DD is 36 months and has taken up wanting a feed again after weaning herself at 22 months and while at the moment I am happy with a little feed before bed time, I do have to tell 'fibs' at other points in the day - feeding DS in the day is enough for me (maybe if she had continued feeding right through would be different but after not feeding for 9 months or so is different, especially because she is no longer very good at it).