Unnecessarily unkind.
Stop sending presents. Put the money away for them instead, to be given to them either by you, or through your wills, when they are adult, if possible with a loving and explanatory letter.
There are various ways you could do this without involving their parents, such as a special bank account in your name but the contents of it left to your grandsons in your will.
Sometimes it isn't possible to show our love when the grandchildren are growing up, but they can find out later that they were cared about. Whatever the parents have told them, a special inheritance will tell a different story.