Since children can be very "inventive", a stronger idea would be to have
a per-film or per-category MD5 hash, made of the film's or category's
name and a PIN. That would require a function "protect recording" or
"protect category", then enter a 4-digit PIN, and the function generates
an MD5 hash over the name and the PIN. The hash is stored in the said
config file, and on the USB stick if requested. VDR could memorize the
user's keys as long as the stick is inserted, and forget everything as
soon as it is removed. The stick could hold all the user's keys so that
she or he is authenticated with a single PIN once the stick is inserted.
[...]