Mums, Dads, parents & carers in our schools. Parents and carers used quite a lot but children allowed to talk freely about, and make cards on special occasions, for Mums and Dads. Sensitivity given to children with non-traditional (known or not known) families, bereaved, etc. Quite a few grandparents involved in school drop-off, childcare and in some cases that the children live with, so grandparents also mentioned frequently where it made sense to do so.
"Grown-ups" doesn't make sense as "grown-ups" are all adults the child will meet and not just their trusted caregivers and primary and secondary familial relationships. There could be safeguarding issues saying "grown-ups" too much when you mean parents or other primary caregivers only. I don't think the school in question has got this quite right, whatever their good intentions were.