Frost is building up in my frost free freezer... what could be the problem? Please note that my grandchildren recently moved in with me... could it be that they open it too much?

Yes that is possible. Another possibility is that the door seal is somewhere damaged.

Freezers that start building up frost often can also have a damaged door seal, through which air is sucked in. The water vapor in the air causes the frost build up.

So first thing to do is a proper inspection of the door seals : check from all sides for breaks in the door seals. If there are, replace the entire door seal.

Another cause can be that the freezer door is not closed properly. I have seen many freezers with some format of lock or sticky duct tape to keep the door closed. Whatever suits the situation !

Success !