If I ever was a child……
This area of litigation is often called challenging a will. A will can of course be challenged if there is something wrong with it or how it was made or how it is to be read.
At common law a willmaker is free to do as they wish. Just like anyone can decide to make a gift of something to someone during their lives.
The common law has been changed by statute in most modern jurisdictions to allow certain eligible people (usually those closely related to the deceased) to be able to claim for better provision out of an estate (if adequate provision has not been made for them in circumstances where they should have been properly provided for).
Societies demanded (and legislated many years ago) to protect the rights of wives and dependent children. This area of Estate Litigation is termed Testator's Family Maintenance and more recently Family Provision.
Part 5 dealt with claims of spouses. This Part deals with the claims of children.
For Family Provision claims in Queensland the following types of children are eligible to claim:-
Grandchildren of the deceased may claim but only if they can establish actual financial dependency on the deceased (i.e. the deceased was financially supporting the grandchild at the time of death).
Very young children that have not been provided for adequately (naturally enough) have very strong cases. Those drafting the family provision legislation using the language “adequate provision is not made from the estate for the proper maintenance and support for the spouse, child…..” clearly had in their minds deserted wives and minor children who would not otherwise be able to properly fend for themselves. Minor children should be properly maintained and supported. They are expensive little things.
For an adult child to be successful they must show a special need or special circumstances that warrant provision being made for them. Commonly, they will say they:-
Adult children who cannot make out a case in these terms usually fail. In saying this the High Court of Australia has discussed the notion of special need or claim in the following terms:-
In some cases a special claim may be found to exist because the applicant has contributed to building up the testator's estate or has helped him in other ways. In other cases a son who has done nothing for his parents may have a special need. This may be because he suffers from some physical or mental infirmity, but it is not necessary for an adult son to show that his earning powers have been impaired by some disability before he can establish a special need for maintenance or support. He may have suffered a financial disaster; he may be unable to obtain employment; he may have a number of dependents who rely on him for support which he cannot adequately provide from his own resources. There are no rigid rules; the question whether adequate provision has been made for the proper maintenance and support of the adult son must depend on all the circumstances –– that is, on all the facts that existed at the date of the death of the testator, whether the testator knew of them or not, and all the eventualities that might at that date reasonably have been foreseen by a testator who knew the facts. 
The High Court has also offered the following:-
a just father’s moral duty is to assist the lame ducks amongst his offspring, provided they be not morally or otherwise undeserving” 
The Court may refuse to make an order in favour of any person whose character or conduct is such as, in the opinion of the court, disentitles him or her to the benefit of an order, or whose circumstances are such as make such refusal reasonable. In my experience there are not many cases where the claim by an executor that a claimant has been guilty of disentitling conduct is successful. It is often difficult for executors and beneficiaries whose entitlements are being reduced because of a claim to accept that this ground cannot be made out. There are often very good reasons why testators have not made provision for a child.
 Hughes v National Trustees Executors and Agency Co of Australasia Ltd (1979) 143 CLR 134 at 147
 Gibbs J citing with approval the statement of Philp J in Re Hatte  SR (Qld) 1 at 26 (at 148)