Just brainstorming…
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Babies could be found e.g. in an encampment of hostile NPCs (who might have stolen it from somewhere else). This kind of “loot” would be valuable as the player would not need to transfer a resource into a new settler.
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Schools might introduce the teacher as a new profession. Schools could require additional items like chairs, a chalkboard, books and pencils. Education might extend the childhood, i.e. it takes longer for you to get your worker… but in exchange educated workers might get an experience-buff and level faster later on (trade-off). Player could get the option to educate all or only selected children.
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To shorten the childhood (get your worker faster) it might be possible to manually promote a teenager (after a certain period of time) into a worker. The young worker would already look like an adult, but get a de-buff for a certain period of time and work only half as long or half as effective. The de-buff could be couble as long as the remaining childhood would have been. This leads to another trade-off… do I need them now, with a de-buff or do I want them to be proper raised (incl. education) and profit on the long-term.
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Alternative of fast delivery. Early in the game the system might work different (to make the start easier). E.g. a caravan could bring a new worker every time you order one… with some kind of background story of course. This would require some strong value add for the player to change later on. Education-system and buffs could be one motivation. A hard switch in a certain point in time (without the option to chose) might be another approach.