No, in 802.15.4 there is no real parent/child relationship set up, so there isn’t a limitation on the 802.15.4 module on how many children per parent.
In ZigBee, the parent, does set up a parent/child relationship, and the parent will actually keep a child table of its available children. There is limited room on the chip for these tables, and so the child table is set at a specific length depending on how much memory has been allocated for use with the child table.