I'm writing this in the first person, since it's far easier to comprehend than the real scenario.
Let's say I'm 80 with $1mil in the bank. I have two 50 something children, Paul and Mary, and a total of 5 grandkids (one grandkid from Paul, the other four from Mary). Mary is named executor of the estate.
In my will, I specify $450k to each of my children, and the other $100k to be split amongst the five grandchildren at $20k a piece. In my final days, I tell Paul "Don't tell Mary, but really I want your child to get $150k. I know Mary's kids will blow all their money on frivolous things. Once I'm gone and the will is settled, just transfer the extra money to your child."
Of course, the IRS will step in if they see someone gifting any amount over $12k, so is there a legal way to transfer the $150k from Paul to child without the IRS stepping in? It is technically an inheritance, but not one specifically mentioned in the will. I know there's a way for Paul to transfer the money based on his estate, but again, that involves a tax penalty.