Interesting question. I would foresee a problem with what's called the Fertile Octogenarian
rule in law. To de-personalise this so as not to cause any offence, I'll refer to the person making the will as the testator, T. If T dies, leaving two daughters, aged 30 and 50. The 50-yr-old isn't married or in a relationship with a male, and has lived with another woman for many years. The 30-yr old is a nun in an enclosed order. Common sense might say there isn't much likelihood of a grandchild appearing any time soon there.
However, the law still upholds the fertile octogenarian rule, under which the estate won't be distributed until both those daughters have died childless, maybe in their 90's. In effect, the law considers it possible that an 88-yr-old woman, or man, could still become a parent. That's the traditional view.
In consideration of modern medical advances in things like frozen embryos and semen banks etc, the possibilities become open-ended - none of that has yet been tested in court as far as I know.
You can see the dilemma facing a judge if he's asked to interpret the will you want to make, with all those ancient and modern arguments to be considered. I'm sorry that I can't give you any better answer than to consult a solicitor with extensive probate experience, and don't be surprised if the solicitor pulls in a barrister to give a formal opinion and/or to draft the will to ensure it gives effect to your wishes as closely as the law will allow.