Are there restrictions on the freedom to dispose of property upon death (e.g. reserved share)?

Only the descendants of the deceased (children or – if the children have predeceased – their children) are entitled to a reserved share.
Neither the spouse nor the ascendants are entitled to a reserved share.
The reserved share amounts to half of the estate, see Article 4:64 BW. If a descendant invokes his or her reserved share, he or she will no longer be considered as an heir but is a creditor.