I have six kids between the ages of ten and thirty-seven and to "be open and honest with them that there are guns in it and hammer home the point that they are not to ever touch it" is usually the best approach from my experience, especially with young children who haven't fully developed a real concept of danger and it's effects. Believe me, at that age they know a gun is dangerous only because daddy said so, not because they reasoned out for themselves how a gun works, what damage a bullet does to a human body and how that damage can directly impact them if a gun is misused.
OTOH, children the age of your two require a different approach than the one you'll need when they become teens. By the time your kids are that age, they will have to decide for themselves what's right and wrong since trying to "protect" a teen from his/her self is more often than not an exercise in futility for EVERYONE involved. Luckily, you have a few years yet before you reach that point and by then, if you feel your guns must still be locked away to keep your kids from handling them in an unsafe or unwise manner, you've already lost the battle.