Sending encrypted mail by default (nothing to turn on)

Hello. I have set up K-9 mail with my nephews and nieces, and I want encryption enabled by default when they send email to me.

I can set up encryption by default in Thunderbird (on my end). Is such a thing possible on K-9 so that no unencrypted mails get sent (and nothing needs to be turned on, ever), because I am still getting messages from them that are sometimes unencrypted. This defeats the purpose of using encryption.

I am using the latest available K-9 on f-droid.org along with the latest available OpenKeychain on f-droid.org. Is there a way to make it impossible for them to accidentally send me a plain text email from K-9? Thank you!