Associate "default" profile/account with Device?

Since PLEX allows you to have multiple accounts, and it records associated devices in the system settings. Would it be possible to link a “default account” to the device that is connecting. Now that we have profiles turned on for the kids, my wife finds it a bit exasperating to have to constantly select the primary account on a player that is only used by adults. whereas the kids could be defaulted to theirs.

Which Plex Client are you using?
Most Clients have the setting “Automatically Login” which, if checked, will keep the last logged in user logged in unless that users logs out.

Never seen that on ROKU; do you know where that option can be found?

See this link:
https://support.plex.tv/hc/en-us/articles/204275243-Settings-Plex-for-Roku

and make sure Automatically Sign In is checked