WOW! A tough question when dealing kids. First, thanks for your efforts to go the extra step.
However, it’s not really practical when just dealing with LibreOffice or other Office packages. Base is not a database but a front end to a database. The database that comes with Base is a stripped down version of HSQLDB (HyperSQL Database). This version does not allow for password protection or encryption. You can get and install (at no cost) a full version of HSQLDB or other databases (MySQL, PostgreSQL, etc) but that might entail quite a bit of work installing and setting up just for what you want to accomplish. In fact, just to be clear, for what you are talking about you don’t even need Base. Just a database. But again, maybe more trouble than the reward.
Encryption would be tough to demonstrate anyway. It’s more of a substitution which is difficult to actually demonstrate. Simple encryption would be like Morse code - configuration of dots & dashes which actually mean something else. Password protection for a database is just like signing on to an e-mail account or opening a door with a key.
I think it may be easier to use examples than an actual demonstration for encryption (decoder ring or substitution chart where Q = T etc). Sorry, but I can’t think of an easy way.