These answers are insensitive and stupid when a serious question has been asked.
The total tax free amount for IHT - for the two of them - is �624000. All the rest will be taxed at 40%. It is best not to distribute all the money to the children because if there is nothing left in the estate the tax will still have to be paid, & the children who have received the money will have to pay it.
BUT there are various somewhat complex schemes involving trusts which just might reduce the tax burden. If they want to explore this possibility they will have to consult a specialist solicitor - one who is a member of the Society of Trust & Estate Practitioners (see their website).