For Rav Fleischman Shlit”a,
Grandparents intend to purchase tefillin that don’t meet the level of hiddur the parents desire for their son. They either can’t really afford more, will not understand the need to pay more or this is what they’ve bought for other grandchildren without any comment from the other parents (siblings of the parents of the present bar mitzvah boy). The parents are concerned that the grandparents may be upset or humiliated if they ask to chip in for a better pair.
May the parents discreetly contact the sofer and pay the difference to have him upgrade the tefillin?
Here is R’ Fleishman’s Shlita”s answer:
Yes-there is no problem of geneivas da’as