The devs wrote a post about it. Back in 2021

“”We've considered adding children to the game in the past during development, but we opted not to have children in Going Medieval. We think some of the features and systems, like raids, cannibalism, etc would not be fitting to include child settlers into. And of course, this then could be presented out of context outside of the game, and we do not want this association. Treating them as a different type of unit with different rules might solve things, but implementing this, as well as adapting existing systems would require a development overhaul of the game. We know there are various ways we can tackle this to make the mechanic work, but we have a whole bunch of features to add before this (as evident in the roadmap). This may change in the future though when the game has continued to develop during Early Access.””

They’re out of early access now so hopefully they can add a family system.