I found an alternative solution taking some of your suggestions.
This consists in use myChild class constructor to pass as parameter the myParent object property and assign this to myChild.myNumber property.

The only issue is, the myParent object must be known

DEFINE CLASS myChild as myParent OF myparent.prg
	PROCEDURE Init(n as Integer)
		This.myNumber = n
	ENDPROC	
ENDDEFINE

and test prg

*-- testInheritance.prg
o1 = NEWOBJECT("myParent","myparent.prg")
? o1.myNumber
o1.myNumber = 5678
? o1.myNumber
*
* o2 = NEWOBJECT("myChild","mychild.prg",0,lnNumber)
* I had problems to use the parameters clause in NewObject()
*
SET PROCEDURE TO myChild.prg
o2 = CREATEOBJECT("myChild",o1.myNumber)
? o2.myNumber

o1 = null
o2 = null