My best friend sent her son to infant care after 4 months as her maternity leave had ended. As most infant cares take in babies as young as 2 months, the caregivers are trained to interact and care for babies of different ages. At 4 months, there was not much my friend's baby could do either -- not even sit up or crawl. According to my friend, younger babies would usually be placed in cribs or laid out on the playmat/mattresses and the caregiver will interact with them. I also saw pictures of babies in playseats as well as sitting up, supported by the caregivers.
(picture from Google)
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