I think I can assume the freezer isn't a "frost-free".
A short physics lesson; hot air holds more moisture than cold air; hot air will flow from a warm area to a colder area, and the colder area will cause the moisture to condensate and freeze(if colder area below freezing, as a freezer should be), causing frost build up.The more often the freezer is opened, the more frost will build up.
Now for 3 possibilities:1) If it is a self-defrosting type, check your defrost timer control...not working, or not working often enough.2) Door seals damaged and need replacement.3)Make the kids stay out of freezer.