As an Australian and convict descendant I have a couple of things to say.
Firstly, it was far less of a "prison planet" and more what you'd call an exile colony. Do your time (commonly 7 years) and you could even go back to England, if you could manage it (no free return fares though). Even lifers could have their sentences commuted.
No, children of convicts were not criminals. They were just part of the general population.
Mostly, the convicts worked as normal labourers or servants, as a lot of them would have done back in the UK. Some country boy transported for stealing chickens? He'd be put to work on a farm. A clerk who was convicted of larceny? Valuable skills there, get that man to work on the paperwork! Women generally saw no change in their lives, although being outnumbered by the men saw most of them marry up and actually improve their lot.
Convicts that reoffended after transportation were the ones that ended up in durance vile. Keeping in mind that any really serious crimes got you hung anyway. Murderers did not get transported, or sent to secondary punishment.
So, back to the OP.
A child from a penal colony is just a kid with a colourful background, no more or less so than a Spacer child or an Army brat. Choose whatever background skills you feel work, and if you want to embrace the criminal element pursue it as a Rogue or such.
A child born and raised in prison is another matter. That happens in some places, but stepping aside from the real world it's not unreasonable to have a science fiction prison where the kids are added to the prison population until they grow up. If it's a slavery scenario they might not ever be released. Since Prior Careers are more about what skills are earned and what events happen than specific circumstances, "Prisoner" could be used for "slave caste" with Referee permission. Or Colonist. Or Barbarian. Just as "Army" could be local city militia, a mercenary unit, Imperial ground forces or some other kind of organised trained killers. Scouts do NOT have to be IISS if everyone's good. There would be civilian and corporate explorers and couriers too.
So yeah. If you have a cool idea, talk to the Ref and see if they're on board. It's Traveller - no one has to pay points to make it all "fair" (unless you're going with that style, I guess), and background is all stuff the player makes up anyway. Roll your dices and take your chances...
