Server for kids with one account.

Quick and dirty. I have account, oldest has account, other two use our accounts. For personal server can I edit to treat each instance as separate players?

I've hosted MC servers a couple times, either for a couple friends or small discords, and I've always been above board. I know the back end enough to be comfortable with setting up whatever it is my kids or friends want. I do know I can turn off the online function to allow others in but I'd rather not give my kid access to a cracked launcher.

Is there something I can do to the individual computers that would let them connect to only my personal server as a separate account. For easy math let's say I have four computers each with Minecraft installed. My computer has my account, the other three have the oldest child's account on it. I would like to host a server just for us to play together that would let all of them play at the same time and treat each of them as different accounts. Can the same account be logged in to a geyser server with one Java and one bedrock?