The Parsha begins with the Tumah of a Yoledes (a woman who has given birth) and concludes with the laws of a Zav, Zavah and Niddah. Someone with one of these types of Tumah are allowed in the city of Yerushalaim, but they are forbidden to go onto Har Habayis. This means that they too would be unable to participate in Hakhel.
After giving birth to a male, a woman must wait 40 days and bring a Korban, before she can enter the Beis Hamikdash. After giving birth to a female, she must wait 80 days before bringing a Korban when she may once again enter the Beis Hamikdash.
According to the Minchas Chinuch and others, even newborn babies had to participate in Hakhel. They are included in the category of Taf.
So there can be a situation where the mother is unable to enter the Beis Hamikdash for Hakhel, but her newborn child must attend.
Some learn that a newborn child is exempt from Hakhel for this very reason. How can they be expected to join in Hakhel without their mother to take care of them?