Life interest trust disclaimer of remainder interest

A trust was set up in 1987 by the mother (died 2014) with a life interest for one son, remainder interest to other son and daughter in equal shares. There is no default beneficiary.
The opportunity to assign the remainder interests to grandchildren was missed as the life tenant died earlier this year.
The remainderman wish to know if there is a tax efficient way (other than PETS) of passing their interests in the trust fund to their respective children.
If the remaindermen were to disclaim their interests, in the absence of a default beneficiary I assume the trust assets would revert to the Settlor and pass through her Will and end up back with the remaindermen as residuary beneficiaries in equal shares.
My question is - could the son and daughter vary these additional entitlements under their late mother’s Will within 2 years of the death of the life tenant with the ‘reading back’ benefits for IHT and CGT, or could they disclaim them so that the assets would pass to the grandchildren (the Will contains gifts over)? Is one route preferable to the other?
The trust fund is around £500,000 and consists mainly of a share portfolio.
Grateful for any advice from forum members.

On my reading of the question, as the life tenant has died, the remaindermen have become absolutely entitled to their shares of the trust fund so that if they were to execute a document disclaiming their interest, they would in effect be making PETs even if the result was that the trust fund would revert to the settlor’s estate. Therefore why not just make PETs to their children. I appreciate That the income the minor children would receive would be treated as their income for tax purposes but that would also be the case if the disclaimed assets were to pass through the grandmothers estate. 
Looking at the question again, where you ask about varying the mother’s will within two years of the life tenant son’s death, that certainly is not possible.

Mind you, having said all of this, it could be that my mind has been affected by the overeating  of turkey and Christmas pudding!

Happy New Year

Patrick Moroney

Thanks for putting me straight Patrick.
Happy New Year to you too.
Alexandra Taylor