Child is already a subtype of Parent, so your convert call doesn’t change anything. Furthermore, Parent is an abstract type and so cannot be instantiated, i.e. no values can directly have type Parent, though they can be of its concrete subtypes.
You can use invoke to call the more generic method. Alternatively, you can also lean further into multiple dispatch:
_get_height(x::Parent) = x.height / 100
get_height(x::Parent) = _get_height(x)
get_height(x::Child) = _get_height(x) + 0.01