The console command just gives you FM a trait that makes him more fertile. That means if you have enough provinces, he is more likely to have children. Of course there will always be FMs without children, especially if you do not expand continously or if you have lots of FMs.
The roman FMs have invisible traits that make them less fertile. This was implemented to encourage adoptions, as the Romans were not ruled by a single dynasty like the diadochoi factions.
To find out the internal name of a roman FM, you could try a cheat like character_reset, that can't do any harm to your game. If your FM is called "Manivs Aurelivs Cotta", you start typing "Manivs Romanvs", "ManivsA Romanvs" and so on till "ManivsK Romanvs". If you type the name of an existing FM, he will get his movement points reset and be selected, so you know his internal name and can give him traits through the console.
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